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Reclassifications out of Accumulated Other Comprehensive Loss - Summary of Reclassifications out of Accumulated Other Comprehensive Loss (Detail) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]    
Total reclassifications $ (1.9) $ (0.6)
Defined Benefit Pension Plan Items [Member]    
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]    
Total reclassifications (1.9) (0.6)
Reclassification Out of Accumulated Other Comprehensive Income/(Loss) [Member]    
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]    
Total before tax 2.1 0.6
Income tax expense (0.2) (0.0)
Total reclassifications 1.9 0.6
Reclassification Out of Accumulated Other Comprehensive Income/(Loss) [Member] | Defined Benefit Pension Plan Items [Member]    
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]    
Amortization of prior service cost 0.2 (0.6)
Amortization of actuarial net losses $ 1.9 $ 1.2