Revenue from services provided to customers - Schedule of Disaggregation of Brokerage Commissions Revenue (Detail) - JPY (¥) ¥ in Millions	6 Months Ended
Sep. 30, 2025	Sep. 30, 2024
Product Information [Line Items]
Commissions	¥ 205,655	¥ 204,113
Brokerage commissions [Member]
Product Information [Line Items]
Commissions	130,027	131,886
Commissions for distribution of investment trust [Member]
Product Information [Line Items]
Commissions	28,609	34,762
Other commissions [Member]
Product Information [Line Items]
Commissions	¥ 47,019	¥ 37,465

Amount of commission revenue from buying and selling securities on behalf of customers.

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)(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1

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