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, 2020	Mar. 31, 2019	Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivatives designated as net investment hedges	¥ 2	¥ 7	¥ (11)
Foreign exchange contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivatives designated as net investment hedges	¥ 2	¥ 7	¥ (11)