Investment Securities (Carrying Value of Nonmarketable Equity Securities) (Detail) - JPY (¥) ¥ in Millions	Mar. 31, 2025	Mar. 31, 2024
Nonmarketable equity securities \| Measurement alternative
Investment Securities [Line Items]
Measurement alternative balance	¥ 603,633	¥ 483,218

Amount of investment in equity security without readily determinable fair value.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 321
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479536/321-10-50-3

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