v3.26.1
Investment in Securities (Summary of Roll-forwards of Allowance for Credit Losses) (Detail) - JPY (¥)
¥ in Millions
 12 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Mar. 31, 2024
Accounts And Financing Receivable For Credit Losses [Line Items]      
Beginning ¥ 670 ¥ 634 ¥ 144
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded 2,727 141 445
Additional increases to the allowance for credit losses on available-for-sale debt securities that had an allowance recorded in a previous period   32  
Additional increases (decreases) to the allowance for credit losses on AFS debt securities that had an allowance recorded in a previous period, net (695)    
Recoveries of amounts previously written off 755    
Increase (Decrease) from the effects of changes in foreign exchange rates 88 (137) 45
Write-offs charged against the allowance (40)    
Ending 3,505 670 634
Foreign municipal bond securities      
Accounts And Financing Receivable For Credit Losses [Line Items]      
Beginning 245 248 144
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded 528 0 80
Additional increases to the allowance for credit losses on available-for-sale debt securities that had an allowance recorded in a previous period   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    
Recoveries of amounts previously written off 0    
Increase (Decrease) from the effects of changes in foreign exchange rates 25 (3) 24
Write-offs charged against the allowance 0    
Ending 798 245 248
Foreign corporate debt securities      
Accounts And Financing Receivable For Credit Losses [Line Items]      
Beginning 34 0  
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded 31 34  
Additional increases to the allowance for credit losses on available-for-sale debt securities that had an allowance recorded in a previous period   0  
Additional increases (decreases) to the allowance for credit losses on AFS debt securities that had an allowance recorded in a previous period, net (1)    
Recoveries of amounts previously written off 0    
Increase (Decrease) from the effects of changes in foreign exchange rates 0 0  
Write-offs charged against the allowance 0    
Ending 64 34 0
CMBS and RMBS in the Americas      
Accounts And Financing Receivable For Credit Losses [Line Items]      
Beginning 0    
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded 1,902    
Additional increases (decreases) to the allowance for credit losses on AFS debt securities that had an allowance recorded in a previous period, net 0    
Recoveries of amounts previously written off 0    
Increase (Decrease) from the effects of changes in foreign exchange rates 25    
Write-offs charged against the allowance 0    
Ending 1,927 0  
Foreign other asset- backed securities and debt securities      
Accounts And Financing Receivable For Credit Losses [Line Items]      
Beginning 391 386 0
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded 266 107 365
Additional increases to the allowance for credit losses on available-for-sale debt securities that had an allowance recorded in a previous period   32  
Additional increases (decreases) to the allowance for credit losses on AFS debt securities that had an allowance recorded in a previous period, net (694)    
Recoveries of amounts previously written off 755    
Increase (Decrease) from the effects of changes in foreign exchange rates 38 (134) 21
Write-offs charged against the allowance (40)    
Ending ¥ 716 ¥ 391 ¥ 386