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Equity Method Investments (Summary of Equity Method Investments) (Detail) - JPY (¥) ¥ in Millions | Sep. 30, 2025 | Mar. 31, 2025
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Schedule of Equity Method Investments [Line Items] |  |
Equity method investments | ¥ 1,240,679 | ¥ 1,320,015
Investment in corporate entities [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Equity method investments | 825,879 | 973,795
Investment in real estate joint ventures [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Equity method investments | 144,363 | 137,274
Investment in partnerships and other investments [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Equity method investments | ¥ 270,437 | ¥ 208,946

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