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Long-Duration Insurance Contracts Relating to Life Insurance Operations - Schedule of Deferred Policy Acquisition Costs (Detail) - JPY (¥)
¥ in Millions
 6 Months Ended 12 Months Ended
Sep. 30, 2025
Mar. 31, 2025
Deferred Policy Acquisition Cost [Line Items]    
Beginning balance ¥ 315,840 ¥ 305,734
Capitalization 19,007 35,871
Amortization (10,049) (19,215)
Effect of changes in foreign exchange rate 5,650 (742)
Others [1] (9,943) (5,808)
Ending balance 320,505 315,840
Yen-denominated insurance (First Sector)    
Deferred Policy Acquisition Cost [Line Items]    
Beginning balance 89,361 82,341
Capitalization 7,085 13,431
Amortization (3,421) (6,411)
Effect of changes in foreign exchange rate 0 0
Others [1] 0 0
Ending balance 93,025 89,361
Yen-denominated insurance (Third Sector)    
Deferred Policy Acquisition Cost [Line Items]    
Beginning balance 168,689 169,581
Capitalization 3,923 9,180
Amortization (5,040) (10,072)
Effect of changes in foreign exchange rate 0 0
Others [1] 0 0
Ending balance 167,572 168,689
Foreign currency denominated insurance    
Deferred Policy Acquisition Cost [Line Items]    
Beginning balance 57,151 53,812
Capitalization 3,206 6,633
Amortization (1,371) (2,689)
Effect of changes in foreign exchange rate (203) (605)
Others [1] 0 0
Ending balance 58,783 57,151
Single- payment whole life insurance    
Deferred Policy Acquisition Cost [Line Items]    
Beginning balance 639 0
Capitalization 4,793 6,627
Amortization (217) (43)
Effect of changes in foreign exchange rate 5,853 (137)
Others [1] (9,943) (5,808)
Ending balance ¥ 1,125 ¥ 639
[1] Others include adjustments of reinsurance.