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Income and Expenses Relating to Life Insurance Operations - Life Insurance Premiums and Related Investment Income (Parenthetical) (Detail) - JPY (¥) ¥ in Millions | 12 Months Ended
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Mar. 31, 2025 | Mar. 31, 2024 | Mar. 31, 2023
Net Investment Income [Line Items] |  |  |
Trading Securities, Unrealized Holding Gain (Loss) | ¥ 4,422 | ¥ 53,318 | ¥ 18,065
Life insurance related investment income |  |  |
Net Investment Income [Line Items] |  |  |
Trading Securities, Unrealized Holding Gain (Loss) | ¥ 2,577 | ¥ 43,301 | ¥ 851

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