Employees
| Concept | As at 2023-12-31 | As at 2023-12-31 | As at 2023-12-31 | As at 2023-12-31 | As at 2023-12-31 | As at 2023-12-31 | As at 2023-12-31 | As at 2023-12-31 | As at 2023-12-31 | As at 2023-12-31 | As at 2022-12-31 | As at 2022-12-31 | As at 2021-12-31 | As at 2021-12-31 | As at 2020-12-31 | As at 2020-12-31 | As at 2019-12-31 | As at 2019-12-31 | 
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Employees | ||||||||||||||||||
| Information about reporting company | ||||||||||||||||||
| Information about employees of reporting company | ||||||||||||||||||
| Number of employees | 
                                                1
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                20
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                40
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1,083
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                234
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                964
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                40
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                218
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                14
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1,069
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                949
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                934
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1,029
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                983
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                896
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                874
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                964
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
| Average number of temporary employees | 
                                                
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                7
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                3
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                314
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                44
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                288
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                3
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                42
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                3
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                302
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                279
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                285
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                305
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                300
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                282
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                268
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                283
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
| Average age (Years) | — | — | — | — | — | 
                                                44.6
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | 
| Average length of service (Years) | — | — | — | — | — | 
                                                15.4
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | 
| Average annual salary | — | — | — | — | — | 
                                                7,441,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — |