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		3 Months Ended		6 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1],[2]	¥ (20)	¥ 69	¥ 1	¥ 31
Equity contracts [Member]
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1],[2]	78	(22)	128	(61)
Interest rate contracts [Member]
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1],[2]	(177)	5	(266)	87
Credit contracts [Member]
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1],[2]	48	(4)	128	(5)
Foreign exchange contracts [Member]
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1],[2]	15	97	3	(1)
Commodity contracts [Member]
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	[1],[2]	¥ 16	¥ (7)	¥ 8	¥ 11

[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 six and three months ended September 30, 2016 and 2017, these amounts have not been separately presented as net gains (losses) for these non-trading derivatives were not significant.