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, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Disaggregation Of Investment Banking Revenue [Abstract]
Equity underwriting and distribution fees	¥ 1,603	¥ 10,276	¥ 5,316	¥ 20,231
Debt underwriting and distribution fees	4,517	6,123	11,215	13,445
Financial advisory fees	14,279	13,661	28,310	26,776
Other fees	3,790	3,841	6,633	9,152
Total	¥ 24,189	¥ 33,901	¥ 51,474	¥ 69,604

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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.13(b),(d))
-URI https://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

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