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Reclassifications out of Accumulated Other Comprehensive Loss - Summary of Reclassifications out of Accumulated Other Comprehensive Loss (Detail) - USD ($)
$ in Millions
 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]    
Total reclassifications $ (1.3) $ (0.4)
Defined Benefit Pension Plan Items [Member]    
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]    
Total reclassifications (1.3) (0.4)
Reclassification Out of Accumulated Other Comprehensive Income/(Loss) [Member]    
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]    
Total before tax 1.4 0.4
Income tax expense (0.1) (0.0)
Total reclassifications 1.3 0.4
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 0.1 (0.4)
Amortization of actuarial net losses $ 1.3 $ 0.8