Investments - Amortized Cost, Gross Unrealized Gains and Losses, and Fair Value of Available-for-Sale and Held-to-Maturity Securities (Parenthetical) (Detail) - JPY (¥) ¥ in Millions	Mar. 31, 2026	Mar. 31, 2025
Schedule of Investments [Line Items]
Fair value, available-for-sale securities	¥ 21,848,687	¥ 15,058,617
Accrued income
Schedule of Investments [Line Items]
Accrued interest receivables	245,000	239,000
Net of Allowance for Credit Loss
Schedule of Investments [Line Items]
Amortised cost net of allowance for credit loss available for sale securities	0	0
Debt securities \| Negotiable certificates of deposits, asset- backed securities and collateral loan obligations
Schedule of Investments [Line Items]
Fair value, available-for-sale securities	287,547	221,706
Debt securities \| Japanese agency mortgage-backed securities \| Agency mortgage-backed securities
Schedule of Investments [Line Items]
Fair value, available-for-sale securities	281,815	315,536
Debt securities \| Foreign agency mortgage-backed securities \| Agency mortgage-backed securities
Schedule of Investments [Line Items]
Fair value, available-for-sale securities	21	22
Available-for sale and held-to-maturity securities \| Accrued income
Schedule of Investments [Line Items]
Accrued interest receivables	¥ 29,997	¥ 20,845

Amount, after allowance for credit loss, of accrued interest on accounts receivable classified as noncurrent.

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