Employee benefit plans - Information about fair value of plan assets (Parenthetical) (Detail) - JPY (¥) ¥ in Millions	Mar. 31, 2026	Mar. 31, 2025
Amount of net asset value per share as a practical expedient [Member] \| Investment trust funds and other [Member]
Pension plans, postretirement and other employee benefits [Line items]
Fair values of certain plan assets	¥ 38,092	¥ 35,529

Amount of investment within defined benefit plan asset category.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (c)(ii)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-5

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)(ii)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

Name:	us-gaap_DefinedBenefitPlanPlanAssetsInvestmentWithinPlanAssetCategoryAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant