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Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of AOCI) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]      
Pretax total $ (82.9) $ (142.5) $ 14.7
Unrealized Gains (Losses) on Debt Securities      
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]      
Net realized (losses) gains on sales of debt securities (80.9) (136.5) 20.3
Credit losses recognized on debt securities 0.0 0.0 (0.1)
Pretax total (80.9) (136.5) 20.2
Tax effect 19.8 36.2 (5.0)
Pension Benefit Adjustment      
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]      
Amortization of net actuarial loss [1] (2.0) (5.8) (6.8)
Amortization of prior service (cost) credit [1] 0.0 (0.2) 1.3
Pretax total [1] (2.0) (6.0) (5.5)
Tax effect [1] $ 0.5 $ 1.6 $ 1.5
[1] Amounts are components of net periodic cost. See Note 16 Employee Benefit Plans for additional details.