Employees
| Concept | As at 2024-03-31 | As at 2024-03-31 | As at 2024-03-31 | As at 2024-03-31 | As at 2023-03-31 | As at 2023-03-31 | As at 2022-03-31 | As at 2022-03-31 | As at 2021-03-31 | As at 2021-03-31 | As at 2020-03-31 | As at 2020-03-31 | 
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Employees | ||||||||||||
| Information about reporting company | ||||||||||||
| Information about employees of reporting company | ||||||||||||
| Number of employees | 
                                                900
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                900
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                934
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                934
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                879
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                854
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                797
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                816
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                732
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                719
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                654
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                667
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
| Average age (Years) | — | 
                                                41.2
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | 
| Average length of service (Years) | — | 
                                                8.9
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | 
| Average annual salary | — | 
                                                9,035,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — |