Derivative instruments and hedging activities - Schedule of derivative instruments in statement of financial position fair value (Detail) - JPY (¥) ¥ in Billions	Mar. 31, 2021	Mar. 31, 2020
Carrying amount of the hedged liabilities	¥ 1,164	¥ 1,098
Cumulative gains/(losses) of fair value hedging adjustment included in the carrying amount of the hedged liabilities	2	(36)
Long-term borrowings [Member]
Carrying amount of the hedged liabilities	1,164	1,098
Cumulative gains/(losses) of fair value hedging adjustment included in the carrying amount of the hedged liabilities	¥ 2	¥ (36)

Amount of cumulative increase (decrease) in fair value of hedged liability in fair value hedge, attributable to hedged risk.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL109999712-113959

Name:	us-gaap_HedgedLiabilityFairValueHedgeCumulativeIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant