Derivative instruments and hedging activities - Schedule of derivatives used for trading and non-trading purposes (Parenthetical) (Detail) ¥ in Billions	12 Months Ended
Mar. 31, 2021 JPY (¥)
Non Trading Derivatives [Member]
Derivative [Line Items]
Gains (losses) on derivatives used for trading and non-trading purposes	¥ 3

Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5618551-113959

Name:	us-gaap_GainLossOnDerivativeInstrumentsNetPretax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration