Employees
| Concept | As at 2024-03-31 | As at 2024-03-31 | As at 2023-03-31 | As at 2022-03-31 | As at 2021-03-31 | As at 2020-03-31 | 
|---|---|---|---|---|---|---|
| Employees | ||||||
| Information about reporting company | ||||||
| Information about employees of reporting company | ||||||
| Number of employees | 
                                                316
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                316
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                319
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                328
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                330
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                322
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
| Average number of temporary employees | 
                                                31
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                31
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                23
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                20
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                23
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                26
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
| Average age (Years) | 
                                                39.3
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | — | — | — | 
| Average length of service (Years) | 
                                                7.9
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | — | — | — | 
| Average annual salary | 
                                                6,263,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — |