Revenue from services provided to customers - Schedule of Disaggregation of Investment Banking Revenue (Detail) - JPY (¥) ¥ in Millions	3 Months Ended	6 Months Ended
Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Disaggregation Of Investment Banking Revenue [Abstract]
Equity underwriting and distribution fees	¥ 13,097	¥ 1,603	¥ 17,627	¥ 5,316
Debt underwriting and distribution fees	5,211	4,517	10,555	11,215
Financial advisory fees	9,991	14,279	23,615	28,310
Other fees	10,305	3,790	17,953	6,633
Total	¥ 38,604	¥ 24,189	¥ 69,750	¥ 51,474

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),(d))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483589/942-220-S99-1

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration