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Income and Expenses Relating to Life Insurance Operations - Life Insurance Premiums and Related Investment Income (Parenthetical) (Detail) - JPY (¥) ¥ in Millions | 6 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024
Net Investment Income [Line Items] |  |
Trading Securities, Unrealized Holding Gain (Loss) | ¥ 64,276 | ¥ (1,992)
Life insurance related investment income loss |  |
Net Investment Income [Line Items] |  |
Trading Securities, Unrealized Holding Gain (Loss) | ¥ 30,360 | ¥ (997)

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