Revenue from services provided to customers - Schedule of Disaggregation of Investment Banking Revenue (Detail) - JPY (¥) ¥ in Millions	6 Months Ended
Sep. 30, 2025	Sep. 30, 2024
Disaggregation Of Investment Banking Revenue [Abstract]
Equity underwriting and distribution fees	¥ 8,579	¥ 26,669
Debt underwriting and distribution fees	25,287	20,212
Financial advisory fees	35,289	31,912
Other fees	13,756	15,793
Total	¥ 82,911	¥ 94,586

Includes (1) underwriting revenue (the spread between the resale price received and the cost of the securities and related expenses) generated through the purchasing, distributing and reselling of new issues of securities (alternatively, could be a secondary offering of a large block of previously issued securities); and (2) fees earned for mergers, acquisitions, divestitures, restructurings, and other types of financial advisory services.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(13)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(13)(d))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1

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