Obligations Under Guarantees and Other Off-balance Sheet Instruments (Contractual Amounts with Regard to Other Off-balance Sheet Instruments) (Detail) - JPY (¥) ¥ in Billions	Sep. 30, 2025	Mar. 31, 2025
Commitments to extend credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Contractual amounts with regard to other off-balance sheet instruments	¥ 101,018	¥ 96,370
Commercial letters of credit
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Contractual amounts with regard to other off-balance sheet instruments	665	717
Commitments to make investments
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Contractual amounts with regard to other off-balance sheet instruments	¥ 734	¥ 718