Severance Indemnities and Pension Plans (Aggregated Accumulated Benefit Obligations) (Detail) - JPY (¥) ¥ in Millions	Mar. 31, 2025	Mar. 31, 2024
Domestic, Japan \| Pension benefits and SIPs
Defined Benefit Plan Disclosure [Line Items]
Aggregated accumulated benefit obligations	¥ 1,292,334	¥ 1,425,853
Foreign offices and subsidiaries \| Pension benefits and other benefits
Defined Benefit Plan Disclosure [Line Items]
Aggregated accumulated benefit obligations	¥ 171,906	¥ 170,385

Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

Name:	us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant