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Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of AOCI (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2023
Mar. 31, 2022
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]    
Amortization of net actuarial loss $ (0.5)  
Pretax total (5.0) $ (2.3)
Unrealized Gains (Losses) on Securities    
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]    
Net realized (losses) gains on sales of debt securities (4.5) (0.7)
Tax effect 1.1 0.2
Pension Benefit Adjustment    
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]    
Amortization of net actuarial loss [1] (0.5) (1.5)
Amortization of prior service cost [1] 0.0 (0.1)
Pretax total [1] (0.5) (1.6)
Tax effect [1] $ 0.2 $ 0.5
[1] Amounts are components of net periodic cost. See Note 11 Employee Benefit Plans for additional details.