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Derivative Financial Instruments and Hedging (Notional Amounts of Derivative Instruments and Other, Fair Values of Derivative Instruments and Other before Offsetting) (Detail) - JPY (¥)
¥ in Millions
Sep. 30, 2025
Mar. 31, 2025
Derivatives, Fair Value [Line Items]    
Derivative assets, Fair value ¥ 84,038 ¥ 64,170
Derivative liabilities, Fair value 74,391 56,038
Interest rate swap agreements | Derivatives designated as hedging instruments and other    
Derivatives, Fair Value [Line Items]    
Notional amount 660,629 676,691
Interest rate swap agreements | Derivatives designated as hedging instruments and other | Other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, Fair value 17,766 17,788
Interest rate swap agreements | Derivatives designated as hedging instruments and other | Other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative liabilities, Fair value 2,843 2,768
Interest rate swap agreements | Derivatives not designated as hedging instruments    
Derivatives, Fair Value [Line Items]    
Notional amount 1,303 1,477
Interest rate swap agreements | Derivatives not designated as hedging instruments | Other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, Fair value 75 81
Interest rate swap agreements | Derivatives not designated as hedging instruments | Other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative liabilities, Fair value 2 6
Options held/written and other | Derivatives designated as hedging instruments and other    
Derivatives, Fair Value [Line Items]    
Notional amount 85,362 94,608
Options held/written and other | Derivatives designated as hedging instruments and other | Other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, Fair value 7 98
Options held/written and other | Derivatives designated as hedging instruments and other | Other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative liabilities, Fair value 12,589 7,323
Options held/written and other | Derivatives not designated as hedging instruments    
Derivatives, Fair Value [Line Items]    
Notional amount 779,487 582,939
Options held/written and other | Derivatives not designated as hedging instruments | Other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, Fair value 15,871 15,669
Options held/written and other | Derivatives not designated as hedging instruments | Other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative liabilities, Fair value 7,029 6,392
Futures, foreign exchange contracts | Derivatives designated as hedging instruments and other    
Derivatives, Fair Value [Line Items]    
Notional amount 947,525 932,649
Futures, foreign exchange contracts | Derivatives designated as hedging instruments and other | Other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, Fair value 2,661 9,515
Futures, foreign exchange contracts | Derivatives designated as hedging instruments and other | Other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative liabilities, Fair value 36,648 25,445
Futures, foreign exchange contracts | Derivatives not designated as hedging instruments    
Derivatives, Fair Value [Line Items]    
Notional amount 1,496,880 1,592,590
Futures, foreign exchange contracts | Derivatives not designated as hedging instruments | Other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, Fair value 31,552 11,449
Futures, foreign exchange contracts | Derivatives not designated as hedging instruments | Other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative liabilities, Fair value 14,706 13,942
Foreign currency swap agreements | Derivatives designated as hedging instruments and other    
Derivatives, Fair Value [Line Items]    
Notional amount 461,350 303,060
Foreign currency swap agreements | Derivatives designated as hedging instruments and other | Other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, Fair value 16,098 9,570
Foreign currency swap agreements | Derivatives designated as hedging instruments and other | Other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative liabilities, Fair value 573 159
Foreign currency long-term debt | Derivatives designated as hedging instruments and other    
Derivatives, Fair Value [Line Items]    
Notional amount 92,444 250,702
Foreign currency long-term debt | Derivatives designated as hedging instruments and other | Other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, Fair value 0 0
Foreign currency long-term debt | Derivatives designated as hedging instruments and other | Other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative liabilities, Fair value 0 0
Credit derivatives held/written | Derivatives not designated as hedging instruments    
Derivatives, Fair Value [Line Items]    
Notional amount 1,357 1,000
Credit derivatives held/written | Derivatives not designated as hedging instruments | Other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, Fair value 8 0
Credit derivatives held/written | Derivatives not designated as hedging instruments | Other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative liabilities, Fair value ¥ 1 ¥ 3