Key Financial Data Of Reporting Company
| Concept | 2023-01-01 to 2023-12-31 | As at 2023-12-31 | 2022-01-01 to 2022-12-31 | As at 2022-12-31 | As at 2021-12-31 | 2021-01-01 to 2021-12-31 | 2020-01-01 to 2020-12-31 | As at 2020-12-31 | As at 2019-12-31 | 2019-01-01 to 2019-12-31 | 
|---|---|---|---|---|---|---|---|---|---|---|
| Key financial data of reporting company | ||||||||||
| Business results of reporting company | ||||||||||
| Net sales | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                10,672,305,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                9,305,952,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                7,855,189,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
| Operating revenue | 
                                                1,369,551,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                1,466,689,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
| Ordinary profit (loss) | 
                                                120,158,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                238,487,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                436,111,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                291,825,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                334,033,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
| Profit (loss) | 
                                                73,191,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                175,952,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                311,011,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                212,261,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                239,278,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
| Equity in earnings (losses) of affiliates if equity method is applied | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
| Share capital | — | 
                                                319,256,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                315,771,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                308,725,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                302,650,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                295,975,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
| Total number of issued shares | — | 
                                                3,885,000
                                                
                                                    
                                                        xbrli:shares
                                                    
                                                
                                             | — | 
                                                3,865,200
                                                
                                                    
                                                        xbrli:shares
                                                    
                                                
                                             | 
                                                3,824,800
                                                
                                                    
                                                        xbrli:shares
                                                    
                                                
                                             | — | — | 
                                                1,880,000
                                                
                                                    
                                                        xbrli:shares
                                                    
                                                
                                             | 
                                                1,844,400
                                                
                                                    
                                                        xbrli:shares
                                                    
                                                
                                             | — | 
| Net assets | — | 
                                                1,451,788,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                1,499,929,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                1,467,132,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                1,226,369,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                1,039,121,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
| Total assets | — | 
                                                3,013,606,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                2,976,646,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                3,312,015,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                3,020,572,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                2,512,128,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
| Net assets per share | — | 
                                                376.56
                                                
                                             | — | 
                                                389.54
                                                
                                             | 
                                                381.36
                                                
                                             | — | — | 
                                                325.20
                                                
                                             | 
                                                281.69
                                                
                                             | — | 
| Dividend paid per share | 
                                                32.00
                                                
                                             | — | 
                                                30.00
                                                
                                             | — | — | 
                                                32.00
                                                
                                             | 
                                                27.50
                                                
                                             | — | — | 
                                                25.00
                                                
                                             | 
| Interim dividend paid per share | 
                                                17.00
                                                
                                             | — | 
                                                15.00
                                                
                                             | — | — | 
                                                20.00
                                                
                                             | 
                                                12.50
                                                
                                             | — | — | 
                                                15.00
                                                
                                             | 
| Basic earnings (loss) per share | 
                                                19.10
                                                
                                             | — | 
                                                45.85
                                                
                                             | — | — | 
                                                81.82
                                                
                                             | 
                                                56.95
                                                
                                             | — | — | 
                                                65.01
                                                
                                             | 
| Diluted earnings per share | 
                                                18.91
                                                
                                             | — | 
                                                44.86
                                                
                                             | — | — | 
                                                79.30
                                                
                                             | 
                                                54.77
                                                
                                             | — | — | 
                                                61.33
                                                
                                             | 
| Equity-to-asset ratio | — | 
                                                0.480
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | 
                                                0.502
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                0.440
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | 
                                                0.406
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                0.414
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | 
| Rate of return on equity | 
                                                0.050
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | 
                                                0.119
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | 
                                                0.232
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                0.187
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | 
                                                0.255
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
| Price-earnings ratio | 
                                                39.79
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | 
                                                23.47
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | 
                                                49.38
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                19.80
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | 
                                                21.92
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
| Payout ratio | 
                                                1.676
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | 
                                                0.654
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | 
                                                0.270
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                0.243
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | 
                                                0.135
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
| Net cash provided by (used in) operating activities | 
                                                344,370,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                -118,912,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                385,588,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                351,147,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                189,763,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
| Net cash provided by (used in) investing activities | 
                                                -17,623,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                -33,307,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                -23,347,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                98,778,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                -96,632,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
| Net cash provided by (used in) financing activities | 
                                                -116,933,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                -212,338,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                -54,901,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                -29,368,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                -40,623,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
| Cash and cash equivalents | — | 
                                                1,416,013,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
                                                1,206,199,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                1,570,757,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                1,263,417,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                842,860,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
| Number of employees | — | 
                                                134
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | 
                                                138
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                115
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | 
                                                96
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                90
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | 
| Average number of temporary employees | — | 
                                                
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | 
                                                
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | 
                                                
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | 
| Total shareholder return | — | 
                                                1.061
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | 
                                                1.406
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                4.968
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | 
                                                1.405
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1.750
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | 
| Total return on share price index | — | 
                                                1.010
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | 
                                                1.000
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1.280
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | 
                                                1.510
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1.125
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — |