Derivative Financial Instruments and Hedge Accounting - Notional Amounts and Fair Value of Credit Derivative Portfolio by Type of Counter Party (Detail) - Trading [member] - JPY (¥) ¥ in Millions	Mar. 31, 2026	Mar. 31, 2025
Disclosure of detailed information about financial instruments [line items]
Notional amount	¥ 2,295,916,743	¥ 1,908,888,898
Credit derivatives [member]
Disclosure of detailed information about financial instruments [line items]
Notional amount	4,484,549	3,349,082
Credit derivatives [member] \| Protection purchased [member]
Disclosure of detailed information about financial instruments [line items]
Notional amount	2,665,553	1,943,776
Credit derivatives [member] \| Protection purchased [member] \| Banks and broker dealers [member]
Disclosure of detailed information about financial instruments [line items]
Notional amount	2,665,553	1,943,776
Credit derivatives [member] \| Protection sold [member]
Disclosure of detailed information about financial instruments [line items]
Notional amount	1,818,997	1,405,306
Credit derivatives [member] \| Protection sold [member] \| Banks and broker dealers [member]
Disclosure of detailed information about financial instruments [line items]
Notional amount	¥ 1,818,997	¥ 1,405,306