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, 2021	Mar. 31, 2020	Mar. 31, 2019
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1],[2]	¥ 229	¥ (161)	¥ 13
Equity contracts [Member]
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1],[2]	26	93	(32)
Interest rate contracts [Member]
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1],[2]	254	(192)	104
Credit contracts [Member]
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1],[2]	(90)	(118)	(19)
Foreign exchange contracts [Member]
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1],[2]	(11)	57	(50)
Commodity contracts [Member]
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1],[2]	¥ 50	¥ (1)	¥ 10

[1] Each derivative classification includes derivatives referencing multiple risk components. For example, interest rates contracts include complex derivatives referencing interest rate risk as well as foreign exchange risk or other factors such as prepayment rates. Credit contracts include credit default swaps as well as derivatives referencing corporate and government securities.

[2] Includes net gains (losses) on derivatives used for non-trading purposes which are not designated as fair value or net investment hedges. For the years ended March 31, 2019 and 2020, these amounts were not significant. For the year ended March 31, 2021, net losses for these non-trading derivatives were ¥3 billion.