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Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of AOCI) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]      
Pretax total $ (337.4) $ (82.9) $ (142.5)
Unrealized Gains (Losses) on Debt Securities      
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]      
Net realized losses on sales of debt securities (335.1) (80.9) (136.5)
Tax effect 89.5 19.8 36.2
Pension Benefit Adjustment      
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]      
Amortization of net actuarial loss [1] (2.3) (2.0) (5.8)
Amortization of prior service cost [1] 0.0 0.0 (0.2)
Pretax total [1] (2.3) (2.0) (6.0)
Tax effect [1] $ 0.6 $ 0.5 $ 1.6
[1] Amounts are components of net periodic cost. See Note 16 Employee Benefit Plans for additional details.