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Accumulated Other Comprehensive Income (Loss) (Reclassifications Out of AOCI) (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]        
Amortization of net actuarial loss     $ (1.5)  
Pretax total $ (5.5) $ (51.5) (13.9) $ (62.2)
Unrealized Gains (Losses) on Securities        
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]        
Net realized losses on sales of debt securities (5.1) (50.1) (12.4) (57.7)
Tax effect 1.2 12.9 3.1 14.6
Pension Benefit Adjustment        
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]        
Amortization of net actuarial loss [1] (0.4) (1.4) (1.5) (4.3)
Amortization of prior service cost [1] 0.0 0.0 0.0 (0.2)
Pretax total [1] (0.4) (1.4) (1.5) (4.5)
Tax effect [1] $ 0.0 $ 0.4 $ 0.4 $ 1.2
[1] Amounts are components of net periodic cost. See Note 12 Employee Benefit Plans for additional details.