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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, 2024
Mar. 31, 2023
Mar. 31, 2022
Accounts And Financing Receivable For Credit Losses [Line Items]      
Beginning ¥ 144 ¥ 153 ¥ 120
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded 445   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 45 12 12
Ending 634 144 153
Foreign municipal bond securities      
Accounts And Financing Receivable For Credit Losses [Line Items]      
Beginning 144 132 120
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded 80   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 24 12 12
Ending 248 144 132
Japanese other asset- backed securities and debt securities      
Accounts And Financing Receivable For Credit Losses [Line Items]      
Beginning 0 21 0
Additions to the allowance for credit losses on available-for-sale debt securities for which credit losses were not previously recorded     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 0
Ending   0 ¥ 21
Foreign other asset- backed securities and debt securities [Member]      
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 365    
Increase (Decrease) from the effects of changes in foreign exchange rates 21    
Ending ¥ 386 ¥ 0