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Investment in Securities (Information about Available-for-Sale and Held-to-Maturity Securities with Gross Unrealized Losses and Length of Time that Individual Securities Have Been in Continuous Unrealized Loss Position) (Detail) - JPY (¥) ¥ in Millions | Sep. 30, 2025 | Mar. 31, 2025
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Japanese and foreign government bond securities |  |
Investments, Unrealized Loss Position [Line Items] |  |
Available-for-sale debt Securities, Less than 12 months Fair value | ¥ 152,319 | ¥ 132,283
Available-for-sale debt Securities, Less than 12 months Gross unrealized losses | (19,752) | (9,785)
Available-for-sale debt Securities, 12 months or more Fair value | 894,100 | 913,742
Available-for-sale debt Securities, 12 months or more Gross unrealized losses | (529,457) | (419,686)
Available-for-sale debt Securities, Total Fair value | 1,046,419 | 1,046,025
Available-for-sale debt Securities, Total Gross unrealized losses | (549,209) | (429,471)
Japanese and foreign government bond securities | Allowance for credit losses not recorded |  |
Investments, Unrealized Loss Position [Line Items] |  |
Available-for-sale debt Securities, Less than 12 months Fair value | 152,319 | 132,283
Available-for-sale debt Securities, Less than 12 months Gross unrealized losses | (19,752) | (9,785)
Available-for-sale debt Securities, 12 months or more Fair value | 894,100 | 913,742
Available-for-sale debt Securities, 12 months or more Gross unrealized losses | (529,457) | (419,686)
Available-for-sale debt Securities, Total Fair value | 1,046,419 | 1,046,025
Available-for-sale debt Securities, Total Gross unrealized losses | (549,209) | (429,471)
Japanese prefectural and foreign municipal bond securities |  |
Investments, Unrealized Loss Position [Line Items] |  |
Available-for-sale debt Securities, Less than 12 months Fair value | 90,381 | 95,936
Available-for-sale debt Securities, Less than 12 months Gross unrealized losses | (1,971) | (3,409)
Available-for-sale debt Securities, 12 months or more Fair value | 191,160 | 224,679
Available-for-sale debt Securities, 12 months or more Gross unrealized losses | (32,797) | (31,734)
Available-for-sale debt Securities, Total Fair value | 281,541 | 320,615
Available-for-sale debt Securities, Total Gross unrealized losses | (34,768) | (35,143)
Japanese prefectural and foreign municipal bond securities | Allowance for credit losses not recorded |  |
Investments, Unrealized Loss Position [Line Items] |  |
Available-for-sale debt Securities, Less than 12 months Fair value | 90,381 | 94,691
Available-for-sale debt Securities, Less than 12 months Gross unrealized losses | (1,971) | (3,325)
Available-for-sale debt Securities, 12 months or more Fair value | 186,271 | 220,950
Available-for-sale debt Securities, 12 months or more Gross unrealized losses | (32,553) | (31,573)
Available-for-sale debt Securities, Total Fair value | 276,652 | 315,641
Available-for-sale debt Securities, Total Gross unrealized losses | (34,524) | (34,898)
Corporate debt securities |  |
Investments, Unrealized Loss Position [Line Items] |  |
Available-for-sale debt Securities, Less than 12 months Fair value | 113,776 | 152,094
Available-for-sale debt Securities, Less than 12 months Gross unrealized losses | (4,303) | (5,196)
Available-for-sale debt Securities, 12 months or more Fair value | 407,541 | 427,837
Available-for-sale debt Securities, 12 months or more Gross unrealized losses | (131,604) | (113,802)
Available-for-sale debt Securities, Total Fair value | 521,317 | 579,931
Available-for-sale debt Securities, Total Gross unrealized losses | (135,907) | (118,998)
Corporate debt securities | Allowance for credit losses not recorded |  |
Investments, Unrealized Loss Position [Line Items] |  |
Available-for-sale debt Securities, Less than 12 months Fair value | 113,111 | 149,367
Available-for-sale debt Securities, Less than 12 months Gross unrealized losses | (4,291) | (5,128)
Available-for-sale debt Securities, 12 months or more Fair value | 407,541 | 427,837
Available-for-sale debt Securities, 12 months or more Gross unrealized losses | (131,604) | (113,802)
Available-for-sale debt Securities, Total Fair value | 520,652 | 577,204
Available-for-sale debt Securities, Total Gross unrealized losses | (135,895) | (118,930)
CMBS and RMBS in the Americas |  |
Investments, Unrealized Loss Position [Line Items] |  |
Available-for-sale debt Securities, Less than 12 months Fair value | 17,558 | 16,940
Available-for-sale debt Securities, Less than 12 months Gross unrealized losses | (36) | (103)
Available-for-sale debt Securities, 12 months or more Fair value | 13,035 | 15,817
Available-for-sale debt Securities, 12 months or more Gross unrealized losses | (956) | (777)
Available-for-sale debt Securities, Total Fair value | 30,593 | 32,757
Available-for-sale debt Securities, Total Gross unrealized losses | (992) | (880)
CMBS and RMBS in the Americas | Allowance for credit losses not recorded |  |
Investments, Unrealized Loss Position [Line Items] |  |
Available-for-sale debt Securities, Less than 12 months Fair value | 17,558 | 16,940
Available-for-sale debt Securities, Less than 12 months Gross unrealized losses | (36) | (103)
Available-for-sale debt Securities, 12 months or more Fair value | 13,035 | 15,817
Available-for-sale debt Securities, 12 months or more Gross unrealized losses | (956) | (777)
Available-for-sale debt Securities, Total Fair value | 30,593 | 32,757
Available-for-sale debt Securities, Total Gross unrealized losses | (992) | (880)
Other asset-backed securities and debt securities |  |
Investments, Unrealized Loss Position [Line Items] |  |
Available-for-sale debt Securities, Less than 12 months Fair value | 12,734 | 56,671
Available-for-sale debt Securities, Less than 12 months Gross unrealized losses | (66) | (411)
Available-for-sale debt Securities, 12 months or more Fair value | 36,697 | 35,183
Available-for-sale debt Securities, 12 months or more Gross unrealized losses | (8,069) | (6,966)
Available-for-sale debt Securities, Total Fair value | 49,431 | 91,854
Available-for-sale debt Securities, Total Gross unrealized losses | (8,135) | (7,377)
Other asset-backed securities and debt securities | Allowance for credit losses not recorded |  |
Investments, Unrealized Loss Position [Line Items] |  |
Available-for-sale debt Securities, Less than 12 months Fair value | 12,734 | 56,671
Available-for-sale debt Securities, Less than 12 months Gross unrealized losses |  | (340)
Available-for-sale debt Securities, Less than 12 months Gross unrealized losses | 5 |
Available-for-sale debt Securities, 12 months or more Fair value | 35,168 | 34,868
Available-for-sale debt Securities, 12 months or more Gross unrealized losses | (7,411) | (6,553)
Available-for-sale debt Securities, Total Fair value | 47,902 | 91,539
Available-for-sale debt Securities, Total Gross unrealized losses | (7,406) | (6,893)
Available-for-sale debt securities |  |
Investments, Unrealized Loss Position [Line Items] |  |
Available-for-sale debt Securities, Less than 12 months Fair value | 386,768 | 453,924
Available-for-sale debt Securities, Less than 12 months Gross unrealized losses | (26,128) | (18,904)
Available-for-sale debt Securities, 12 months or more Fair value | 1,542,533 | 1,617,258
Available-for-sale debt Securities, 12 months or more Gross unrealized losses | (702,883) | (572,965)
Available-for-sale debt Securities, Total Fair value | 1,929,301 | 2,071,182
Available-for-sale debt Securities, Total Gross unrealized losses | (729,011) | (591,869)
Available-for-sale debt securities | Allowance for credit losses not recorded |  |
Investments, Unrealized Loss Position [Line Items] |  |
Available-for-sale debt Securities, Less than 12 months Fair value | 386,103 | 449,952
Available-for-sale debt Securities, Less than 12 months Gross unrealized losses | (26,045) | (18,681)
Available-for-sale debt Securities, 12 months or more Fair value | 1,536,115 | 1,613,214
Available-for-sale debt Securities, 12 months or more Gross unrealized losses | (701,981) | (572,391)
Available-for-sale debt Securities, Total Fair value | 1,922,218 | 2,063,166
Available-for-sale debt Securities, Total Gross unrealized losses | ¥ (728,026) | ¥ (591,072)

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