v3.26.3
Accumulated Other Comprehensive Income (Loss) - Other Comprehensive Income (Loss) Reclassification Adjustments (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Jun. 30, 2026
Mar. 31, 2026
Sep. 30, 2025
Jun. 30, 2025
Mar. 31, 2025
Sep. 30, 2026
Sep. 30, 2025
Accumulated Other Comprehensive Income Loss [Line Items]                
Pretax change before reclassifications $ (484.3)     $ 79.8     $ (604.1) $ 262.2
Reclassifications out of AOCI 1.9     2.5     5.7 10.3
Tax effect 121.2     (22.9)     145.9 (62.3)
Total other comprehensive income (loss), net of tax (361.2) $ (25.5) $ (65.8) 59.4 $ 54.2 $ 96.6 (452.5) 210.2
Unrealized Gains (Losses) on Debt Securities                
Accumulated Other Comprehensive Income Loss [Line Items]                
Pretax change before reclassifications (486.7)     91.7     (586.8) 234.5
Reclassifications out of AOCI 1.4     2.0     4.2 8.9
Tax effect 121.3     (22.9)     146.0 (61.3)
Total other comprehensive income (loss), net of tax (364.0)     70.8     (436.6) 182.1
Foreign Currency Translation Adjustment                
Accumulated Other Comprehensive Income Loss [Line Items]                
Pretax change before reclassifications 2.4     (11.9)     (17.3) 27.7
Reclassifications out of AOCI 0.0     0.0     0.0 0.0
Tax effect 0.0     0.2     0.3 (0.6)
Total other comprehensive income (loss), net of tax 2.4     (11.7)     (17.0) 27.1
Pension Benefit Adjustment                
Accumulated Other Comprehensive Income Loss [Line Items]                
Pretax change before reclassifications 0.0     0.0     0.0 0.0
Reclassifications out of AOCI 0.5     0.5     1.5 1.4
Tax effect (0.1)     (0.2)     (0.4) (0.4)
Total other comprehensive income (loss), net of tax $ 0.4     $ 0.3     $ 1.1 $ 1.0