Fair value measurements - Geographic allocations of trading assets related to government, agency and municipal securities (Parenthetical) (Detail) - JPY (¥) ¥ in Millions	Sep. 30, 2025	Mar. 31, 2025
Fair value, concentration of credit risk [Line items]
Non-trading debt securities	¥ 593,272	¥ 485,290
Government, agency and municipal securities [Member]
Fair value, concentration of credit risk [Line items]
Non-trading debt securities	¥ 501,000	¥ 313,000

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 21
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-21

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 20
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-20

Reference 3: http://www.xbrl.org/2003/role/recommendedDisclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 23
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-23

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