Derivative instruments and hedging activities - Schedule of gains (losses) from derivatives and non-derivatives designated as net investment hedges (Detail) - Net investment hedges [Member] - JPY (¥) ¥ in Billions	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivatives and non-derivatives designated as net investment hedges	¥ 6	¥ 7	¥ 7
Foreign Exchange Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivatives and non-derivatives designated as net investment hedges	¥ 6	¥ 7	¥ 7