Key Financial Data Of Reporting Company
| Concept | 2024-04-01 to 2024-09-30 | As at 2024-09-30 | As at 2024-03-31 | 2023-04-01 to 2024-03-31 | 2023-04-01 to 2023-09-30 | As at 2023-09-30 | 
|---|---|---|---|---|---|---|
| Key financial data of reporting company | ||||||
| Business results of reporting company | ||||||
| Ordinary profit (loss) | 
                                                621,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                1,354,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                236,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
| Profit (loss) | 
                                                382,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                845,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                149,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
| Equity in earnings (losses) of affiliates if equity method is applied | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
| Share capital | — | 
                                                1,581,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                1,581,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                1,581,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
| Total number of issued shares | — | 
                                                12,000,000
                                                
                                                    
                                                        xbrli:shares
                                                    
                                                
                                             | 
                                                12,000,000
                                                
                                                    
                                                        xbrli:shares
                                                    
                                                
                                             | — | — | 
                                                12,000,000
                                                
                                                    
                                                        xbrli:shares
                                                    
                                                
                                             | 
| Net assets | — | 
                                                11,836,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                11,630,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                11,004,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
| Total assets | — | 
                                                20,018,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                20,326,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                18,443,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
| Dividend paid per share | 
                                                11.00
                                                
                                             | — | — | 
                                                22.00
                                                
                                             | 
                                                8.00
                                                
                                             | — | 
| Basic earnings (loss) per share | 
                                                32.01
                                                
                                             | — | — | 
                                                70.70
                                                
                                             | 
                                                12.48
                                                
                                             | — | 
| Diluted earnings per share |  | — | — |  |  | — | 
| Equity-to-asset ratio | — | 
                                                0.591
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                0.572
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | 
                                                0.597
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
| Net cash provided by (used in) operating activities | 
                                                335,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                -586,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                358,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
| Net cash provided by (used in) investing activities | 
                                                -7,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                -52,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                -39,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
| Net cash provided by (used in) financing activities | 
                                                -275,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                -427,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                -222,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | 
| Cash and cash equivalents | — | 
                                                4,468,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | 
                                                4,415,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | 
                                                5,579,000,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             |