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Deferred compensation awards - Activity relating to SAR Plan B awards (Detail) - SAR Plan B awards [Member] - ¥ / shares
12 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Outstanding    
Outstanding (number of shares), Beginning 22,338,900  
Outstanding (number of shares), Granted  
Outstanding (number of shares), Exercised (8,966,200)  
Outstanding (number of shares), Forfeited (95,500)  
Outstanding (number of shares), Expired (310,100)  
Outstanding (number of shares), Ending 12,967,100 22,338,900
Outstanding (number of shares), Exercisable, Ending 11,073,000  
Weighted-average grant date fair value per share    
Weighted-average grant date fair value per share, Beginning ¥ 517  
Weighted-average grant date fair value per share, Granted  
Weighted-average grant date fair value per share, Exercised 532  
Weighted-average grant date fair value per share, Forfeited 474  
Weighted-average grant date fair value per share, Expired 457  
Weighted-average grant date fair value per share, Ending 509 ¥ 517
Weighted-average grant date fair value per share, Exercisable, Ending ¥ 514  
Weighted-average remaining life until expiry (years)    
Weighted-average remaining life until expiry (years), Outstanding 2 years 8 months 12 days 3 years 4 months 24 days
Weighted-average remaining life until expiry (years), Exercisable 2 years 1 month 6 days