Collateralized transactions - Schedule of securities received as collateral (Detail) - JPY (¥) ¥ in Billions	Sep. 30, 2025	Mar. 31, 2025
Collateralized Transactions
The fair value of collateral received	¥ 68,052	¥ 64,853
The portion of the above that has been sold (as reported within Trading liabilities in the consolidated balance sheets) or repledged	¥ 50,043	¥ 48,717

Fair value of the collateral, or portion thereof, that has been re-pledged, delivered, or otherwise used.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 1A
-Subparagraph (c)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481420/860-30-50-1A

Name:	us-gaap_FairValueOfSecuritiesReceivedAsCollateralThatHaveBeenResoldOrRepledged
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant