v3.26.3
Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Jun. 30, 2026
Dec. 31, 2025
Jun. 30, 2025
Dec. 31, 2024
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]                
Pretax total accumulated other comprehensive income (loss), beginning balance $ (1,072.0) $ (128.0) $ 130.0 $ (1,809.0)        
Pretax total accumulated other comprehensive income (loss), Investment securities (2,427.0) 362.0 (3,493.0) 2,028.0        
Pretax total accumulated other comprehensive income (loss), Foreign currency translation adjustment 1.0              
OCI, before Reclassifications, before Tax, Attributable to Parent (2,426.0)              
Pretax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities (25.0) 3.0 110.0 (11.0)        
Pretax total accumulated other comprehensive income (loss), Interest expense       (1.0)        
Total reclassification adjustment for amounts realized in net income, pre tax (25.0) 3.0 110.0 (12.0)        
Total other comprehensive income (loss), pre tax (2,401.0) 359.0 (3,603.0) 2,040.0        
Pretax total accumulated other comprehensive income (loss), ending balance (3,473.0) 231.0 (3,473.0) 231.0        
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]                
Total tax (provision) benefit, beginning balance 225.0 33.0 (27.0) 386.0        
Total tax (provision) benefit, Investment securities 510.0 (76.0) 734.0 (426.0)        
Total tax (provision) benefit, Foreign currency translation adjustment 0.0              
OCI, Debt Securities, Available-for-Sale, Gain (Loss), before Adjustment, Tax 510.0              
Total tax (provision) benefit, Net realized gains (losses) on securities 6.0 (1.0) (22.0) 2.0        
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Tax       0.0        
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit 6.0 (1.0) (22.0) 2.0        
Total other comprehensive income (loss), tax (provision) benefit 504.0 (75.0) 756.0 (428.0)        
Total tax (provision) benefit, ending balance 729.0 (42.0) 729.0 (42.0)        
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]                
After tax total accumulated other comprehensive income (loss), beginning balance (847.0) (95.0) 103.0 (1,423.0)        
Stockholders' Equity Attributable to Parent 34,485.0 35,445.0 34,485.0 35,445.0   $ 30,323.0    
After tax total accumulated other comprehensive income (loss), Investment securities (1,917.0) 286.0 (2,759.0) 1,602.0        
Foreign currency translation adjustment 1.0 0.0 0.0 0.0        
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax (1,916.0)              
After tax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities (19.0) 2.0 88.0 (9.0)        
After tax total accumulated other comprehensive income (loss), Interest expense       (1.0)        
Total reclassification adjustment for amounts realized in net income, after tax (19.0) 2.0 88.0 (10.0)        
Total other comprehensive income (loss), after tax (1,897.0) 284.0 (2,847.0) 1,612.0        
After tax total accumulated other comprehensive income (loss), ending balance (2,744.0) 189.0 (2,744.0) 189.0        
Total net unrealized gains (losses) on securities                
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]                
Stockholders' Equity Attributable to Parent (2,730.0) 203.0 (2,730.0) 203.0 $ (832.0) 117.0 $ (81.0) $ (1,408.0)
After tax total accumulated other comprehensive income (loss), Investment securities (1,917.0) 286.0 (2,759.0) 1,602.0        
Foreign currency translation adjustment 0.0              
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax (1,917.0)              
After tax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities (19.0) 2.0 88.0 (9.0)        
After tax total accumulated other comprehensive income (loss), Interest expense       0.0        
Total reclassification adjustment for amounts realized in net income, after tax (19.0) 2.0 88.0 (9.0)        
Total other comprehensive income (loss), after tax (1,898.0) 284.0 (2,847.0) 1,611.0        
Net unrealized losses on forecasted transactions                
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]                
Stockholders' Equity Attributable to Parent (13.0) (13.0) (13.0) (13.0) (13.0) (13.0) (13.0) (14.0)
After tax total accumulated other comprehensive income (loss), Investment securities 0.0 0.0 0.0 0.0        
Foreign currency translation adjustment 0.0              
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax 0.0              
After tax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities 0.0 0.0 0.0 0.0        
After tax total accumulated other comprehensive income (loss), Interest expense       (1.0)        
Total reclassification adjustment for amounts realized in net income, after tax 0.0 0.0 0.0 (1.0)        
Total other comprehensive income (loss), after tax 0.0 0.0 0.0 1.0        
Foreign currency translation adjustment                
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]                
Stockholders' Equity Attributable to Parent (1.0) (1.0) (1.0) (1.0) $ (2.0) $ (1.0) $ (1.0) $ (1.0)
After tax total accumulated other comprehensive income (loss), Investment securities 0.0 0.0 0.0 0.0        
Foreign currency translation adjustment 1.0              
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax 1.0              
After tax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities 0.0 0.0 0.0 0.0        
After tax total accumulated other comprehensive income (loss), Interest expense       0.0        
Total reclassification adjustment for amounts realized in net income, after tax 0.0 0.0 0.0 0.0        
Total other comprehensive income (loss), after tax $ 1.0 $ 0.0 $ 0.0 $ 0.0