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Investment in Securities (Summary of Roll-forwards of Allowance for Credit Losses) (Detail) - JPY (¥)
¥ in Millions
 12 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Mar. 31, 2023
Accounts And Financing Receivable For Credit Losses [Line Items]      
Beginning ¥ 634 ¥ 144 ¥ 153
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded 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     (21)
Increase (Decrease) from the effects of changes in foreign exchange rates (137) 45 12
Ending 670 634 144
Foreign municipal bond securities      
Accounts And Financing Receivable For Credit Losses [Line Items]      
Beginning 248 144 132
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded 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
Increase (Decrease) from the effects of changes in foreign exchange rates (3) 24 12
Ending 245 248 144
Japanese other asset- backed securities and debt securities      
Accounts And Financing Receivable For Credit Losses [Line Items]      
Beginning   0 21
Additional increases (decreases) to the allowance for credit losses on AFS debt securities that had an allowance recorded in a previous period, net     (21)
Increase (Decrease) from the effects of changes in foreign exchange rates     0
Ending     0
Foreign corporate debt securities      
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 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    
Increase (Decrease) from the effects of changes in foreign exchange rates 0    
Ending 34 0  
Foreign other asset- backed securities and debt securities      
Accounts And Financing Receivable For Credit Losses [Line Items]      
Beginning 386 0  
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded 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    
Increase (Decrease) from the effects of changes in foreign exchange rates (134) 21  
Ending ¥ 391 ¥ 386 ¥ 0