Employees
| Concept | As at 2023-12-31 | As at 2022-12-31 | As at 2021-12-31 | As at 2020-12-31 | As at 2019-12-31 | 
|---|---|---|---|---|---|
| Employees | |||||
| Information about reporting company | |||||
| Information about employees of reporting company | |||||
| Number of employees | 
                                                180
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                175
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                173
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                175
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                175
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
| Average number of temporary employees | 
                                                45
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                41
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                46
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                49
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                51
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
| Average age (Years) | 
                                                43.9
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | — | — | 
| Average length of service (Years) | 
                                                11.7
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | — | — | 
| Average annual salary | 
                                                7,563,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — |