Fair Value (Losses (Gains) Recorded as a Result of Changes in Fair Value Measured on a Nonrecurring Basis) (Detail) - JPY (¥) ¥ in Millions	12 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Losses (gains) on change in fair value	¥ 257,606	¥ 141,890
Investment securities
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Losses (gains) on change in fair value	29,409	12,628
Loans
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Losses (gains) on change in fair value	23,210	69,516
Loans \| Loans held for sale
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Losses (gains) on change in fair value	6,903	55,064
Loans \| Collateral dependent loans
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Losses (gains) on change in fair value	16,307	14,452
Premises and equipment
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Losses (gains) on change in fair value	14,490	3,143
Intangible assets
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Losses (gains) on change in fair value	13,057	14,929
Goodwill
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Losses (gains) on change in fair value	150,089	0
Other assets
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Losses (gains) on change in fair value	27,351	41,674
Other assets \| Investments in equity method investees
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Losses (gains) on change in fair value	8,177	19,978
Other assets \| Other
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis [Line Items]
Losses (gains) on change in fair value	¥ 19,174	¥ 21,696