v3.26.3
Investment in Securities (Summary of Roll-forwards of Allowance for Credit Losses) (Detail) - JPY (¥)
¥ in Millions
 6 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Accounts And Financing Receivable For Credit Losses [Line Items]    
Beginning ¥ 3,505 ¥ 670
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded 34 128
Additional increases (decreases) to the allowance for credit losses on AFS debt securities that had an allowance recorded in a previous period, net (56) (26)
Write-offs charged against the allowance (173)  
Recoveries of amounts previously written off 96  
Increase (Decrease) from the effects of changes in foreign exchange rates (52) (1)
Ending 3,354 771
Foreign municipal bond securities    
Accounts And Financing Receivable For Credit Losses [Line Items]    
Beginning 798 245
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded 0 0
Additional increases (decreases) to the allowance for credit losses on AFS debt securities that had an allowance recorded in a previous period, net 0 0
Write-offs charged against the allowance (173)  
Recoveries of amounts previously written off 0  
Increase (Decrease) from the effects of changes in foreign exchange rates (10) (1)
Ending 615 244
Foreign corporate debt securities    
Accounts And Financing Receivable For Credit Losses [Line Items]    
Beginning 64 34
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded 34 0
Additional increases (decreases) to the allowance for credit losses on AFS debt securities that had an allowance recorded in a previous period, net 45 (26)
Write-offs charged against the allowance 0  
Recoveries of amounts previously written off 0  
Increase (Decrease) from the effects of changes in foreign exchange rates 0 0
Ending 143 8
CMBS and RMBS in the Americas    
Accounts And Financing Receivable For Credit Losses [Line Items]    
Beginning 1,927  
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded 0  
Additional increases (decreases) to the allowance for credit losses on AFS debt securities that had an allowance recorded in a previous period, net 0  
Write-offs charged against the allowance 0  
Recoveries of amounts previously written off 0  
Increase (Decrease) from the effects of changes in foreign exchange rates (31)  
Ending 1,896  
Foreign other asset- backed securities and debt securities    
Accounts And Financing Receivable For Credit Losses [Line Items]    
Beginning 716 391
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded 0 128
Additional increases (decreases) to the allowance for credit losses on AFS debt securities that had an allowance recorded in a previous period, net (101) 0
Write-offs charged against the allowance 0  
Recoveries of amounts previously written off 96  
Increase (Decrease) from the effects of changes in foreign exchange rates (11) 0
Ending ¥ 700 ¥ 519