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Investment in Securities (Amortized Cost Basis Amounts, Gross Unrealized Holding Gains, Gross Unrealized Holding Losses and Fair Values of Available-for-Sale Securities and Held-to-Maturity Securities in Each Major Security Type) (Detail) - JPY (¥)
¥ in Millions
Sep. 30, 2025
Mar. 31, 2025
Schedule of Investments [Line Items]    
Available-for-sale debt securities, Amortized cost ¥ 3,394,683 ¥ 3,174,036
Available-for-sale debt securities, Allowance for credit losses (771) (670)
Available-for-sale debt securities, Gross unrealized gains 37,382 25,470
Available-for-sale debt securities, Gross unrealized losses (728,240) (591,199)
Available-for-sale debt securities, Fair value 2,703,054 2,607,637
Japanese and foreign government bond securities    
Schedule of Investments [Line Items]    
Available-for-sale debt securities, Amortized cost 1,607,338 1,520,672
Available-for-sale debt securities, Allowance for credit losses 0 0
Available-for-sale debt securities, Gross unrealized gains 153 1,325
Available-for-sale debt securities, Gross unrealized losses (549,209) (429,471)
Available-for-sale debt securities, Fair value 1,058,282 1,092,526
Japanese prefectural and foreign municipal bond securities    
Schedule of Investments [Line Items]    
Available-for-sale debt securities, Amortized cost 440,413 439,565
Available-for-sale debt securities, Allowance for credit losses (244) (245)
Available-for-sale debt securities, Gross unrealized gains 4,320 2,408
Available-for-sale debt securities, Gross unrealized losses (34,524) (34,898)
Available-for-sale debt securities, Fair value 409,965 406,830
Corporate debt securities    
Schedule of Investments [Line Items]    
Available-for-sale debt securities, Amortized cost 1,051,245 906,297
Available-for-sale debt securities, Allowance for credit losses (8) (34)
Available-for-sale debt securities, Gross unrealized gains 25,781 15,246
Available-for-sale debt securities, Gross unrealized losses (135,899) (118,964)
Available-for-sale debt securities, Fair value 941,119 802,545
CMBS and RMBS in the Americas    
Schedule of Investments [Line Items]    
Available-for-sale debt securities, Amortized cost 131,783 106,578
Available-for-sale debt securities, Allowance for credit losses 0 0
Available-for-sale debt securities, Gross unrealized gains 1,505 1,053
Available-for-sale debt securities, Gross unrealized losses (992) (880)
Available-for-sale debt securities, Fair value 132,296 106,751
Other asset-backed securities and debt securities    
Schedule of Investments [Line Items]    
Available-for-sale debt securities, Amortized cost 163,904 200,924
Available-for-sale debt securities, Allowance for credit losses (519) (391)
Available-for-sale debt securities, Gross unrealized gains 5,623 5,438
Available-for-sale debt securities, Gross unrealized losses (7,616) (6,986)
Available-for-sale debt securities, Fair value ¥ 161,392 ¥ 198,985