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Commitments, contingencies and guarantees - Commitments outstanding (Detail) - JPY (¥)
¥ in Millions
Sep. 30, 2017
Mar. 31, 2017
Commitments to extend credit [Member]    
Commitments [Line Items]    
Commitments outstanding ¥ 1,009,812 ¥ 1,010,257
Commitments to invest [Member]    
Commitments [Line Items]    
Commitments outstanding ¥ 14,200 ¥ 15,194