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Estimated Fair Value of Financial Instruments (Information about Carrying Amount of Financial Instruments Reported in Consolidated Balance Sheets and Related Market or Fair Value) (Parenthetical) (Detail) - JPY (¥) ¥ in Millions | Sep. 30, 2025 | Mar. 31, 2025
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Estimated fair value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Investment funds | ¥ 140,587 | ¥ 118,666

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