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Investment in Securities (Summary of Roll-forwards of Allowance for Credit Losses) (Detail) - JPY (¥)
¥ in Millions
 6 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Accounts And Financing Receivable For Credit Losses [Line Items]    
Beginning ¥ 670 ¥ 634
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded 128 35
Additional increases to the allowance for credit losses on available-for-sale debt securities that had an allowance recorded in a previous period   31
Additional increases (decreases) to the allowance for credit losses on AFS debt securities that had an allowance recorded in a previous period, net (26)  
Increase (Decrease) from the effects of changes in foreign exchange rates (1) (160)
Ending 771 540
Foreign municipal bond securities    
Accounts And Financing Receivable For Credit Losses [Line Items]    
Beginning 245 248
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 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  
Increase (Decrease) from the effects of changes in foreign exchange rates (1) (14)
Ending 244 234
Foreign corporate debt securities    
Accounts And Financing Receivable For Credit Losses [Line Items]    
Beginning 34  
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 (26)  
Increase (Decrease) from the effects of changes in foreign exchange rates 0  
Ending 8  
Foreign other asset- backed securities and debt securities    
Accounts And Financing Receivable For Credit Losses [Line Items]    
Beginning 391 386
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded 128 35
Additional increases to the allowance for credit losses on available-for-sale debt securities that had an allowance recorded in a previous period   31
Additional increases (decreases) to the allowance for credit losses on AFS debt securities that had an allowance recorded in a previous period, net 0  
Increase (Decrease) from the effects of changes in foreign exchange rates 0 (146)
Ending ¥ 519 ¥ 306