Derivative instruments and hedging activities - Schedule of derivatives used for trading and non-trading purposes (Detail) - Derivatives used for trading and non-trading purposes [Member] - JPY (¥) ¥ in Billions		12 Months Ended
		Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2020
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1]	¥ 158	¥ 229	¥ (161)
Equity contracts [Member]
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1]	(36)	26	93
Interest rate contracts [Member]
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1]	198	254	(192)
Credit contracts [Member]
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1]	(118)	(90)	(118)
Foreign exchange contracts [Member]
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1]	27	(11)	57
Commodity contracts [Member]
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1]	¥ 87	¥ 50	¥ (1)

[1] Includes net gains (losses) on derivatives used for non-trading purposes which are not designated as fair value or net investment hedges. For the year ended March 31, 2020, these amounts were not significant. For the year ended March 31, 2021, net losses for these non-trading derivatives were ¥3 billion. For the year ended March 31, 2022, these amounts were not significant.