Accumulated Other Comprehensive Income (Loss) (Changes in Components of Accumulated Other Comprehensive Income (Loss)) (Parenthetical) (Detail) - JPY (¥) ¥ in Millions	6 Months Ended
Sep. 30, 2026	Sep. 30, 2025
Net unrealized gains (losses) on investment in securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net unrealized gains (losses), tax	¥ 36,078	¥ 33,944
Reclassification adjustment included in net income, tax	3,290	1,767
Net unrealized gains (losses) on investments	(145)	(186)
Net unrealized gains (losses) on investments, net of tax	47	28
Impact of changes in policy liability discount rate
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net unrealized gains (losses), tax	(29,198)	(108,460)
Debt valuation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net unrealized gains (losses), tax	(14)	(78)
Reclassification adjustment included in net income, tax	2	1
Defined benefit pension plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net unrealized gains (losses), tax	(4,376)	(2,580)
Reclassification adjustment included in net income, tax	178	69
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net unrealized gains (losses), tax	(1,604)	14,315
Reclassification adjustment included in net income, tax	(599)	3,134
Net unrealized gains (losses) on derivative instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net unrealized gains (losses), tax	2,867	(13)
Reclassification adjustment included in net income, tax	¥ (1,590)	¥ 1,417

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of tax expense (benefit) allocated to other comprehensive income (loss) before reclassification adjustment from accumulated other comprehensive income (loss).

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Amount, after tax and before adjustment, of unrealized holding gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). Excludes unrealized gain (loss) on investment in debt security measured at amortized cost (held-to-maturity) from transfer to available-for-sale.

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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income (loss).

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