Employees
| Concept | As at 2020-03-31 | As at 2020-03-31 | As at 2020-03-31 | As at 2020-03-31 | As at 2020-03-31 | As at 2020-03-31 | As at 2020-03-31 | As at 2019-03-31 | As at 2019-03-31 | As at 2018-03-31 | As at 2018-03-31 | As at 2017-03-31 | As at 2017-03-31 | As at 2016-03-31 | As at 2016-03-31 | 
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Employees | |||||||||||||||
| Information about reporting company | |||||||||||||||
| Information about employees of reporting company | |||||||||||||||
| Number of employees | 
                                                353
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                576
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                2,088
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                690
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                576
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                376
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                669
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                2,082
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                574
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                553
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                2,060
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                557
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1,928
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                545
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1,844
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
| Average age (Years) | — | 
                                                42.0
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Average length of service (Years) | — | 
                                                17.0
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Average annual salary | — | 
                                                6,055,000
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | — | — | — |