Derivative instruments and hedging activities - Schedule of derivative instruments in statement of financial position fair value (Detail) - JPY (¥) ¥ in Billions	Sep. 30, 2025	Mar. 31, 2025
Derivative [Line Items]
Carrying amount of the hedged liabilities	¥ 3,297	¥ 3,057
Cumulative gains of fair value hedging adjustment included in the carrying amount of the hedged liabilities	91	122
Cumulative amount of fair value hedging adjustment remaining for the liabilities which hedge accounting has been discontinued	¥ 3	¥ 3
Long-term borrowings [Member]
Derivative [Line Items]
Hedged Liability, Statement of Financial Position [Extensible Enumeration]	Long-term borrowings	Long-term borrowings
Carrying amount of the hedged liabilities	¥ 3,297	¥ 3,057
Cumulative gains of fair value hedging adjustment included in the carrying amount of the hedged liabilities	91	122
Cumulative amount of fair value hedging adjustment remaining for the liabilities which hedge accounting has been discontinued	¥ 3	¥ 3

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/2009/role/commonPracticeRef
-Topic 815
-SubTopic 20
-Name Accounting Standards Codification
-Section 25
-Paragraph 6A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480682/815-20-25-6A

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480237/815-40-50-5

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Amount of cumulative increase (decrease) in fair value of hedged liability in fair value hedge, attributable to hedged risk, remaining after discontinued hedge. Excludes cumulative basis adjustment related to foreign exchange risk.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4EE
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4EE

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Amount of liability hedged in fair value hedging relationship.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4EE
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4EE

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Amount of cumulative increase (decrease) in fair value of hedged liability in fair value hedge, attributable to hedged risk. Excludes cumulative basis adjustment related to foreign exchange risk.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4EE
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4EE

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- Definition

Indicates line item in statement of financial position that includes hedged liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4EE
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4EE

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