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Income Taxes - Roll-forward of Unrecognized Tax Benefits and Associated Accrued Interest and Penalties (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Income Tax Contingency [Line Items]      
Opening balance at January 1 $ 16.4 $ 14.0 $ 2.5
Additions for tax positions of prior periods 1.0 2.4 12.1
Reductions due to lapsed statute of limitations     (0.6)
Reductions for tax positions of prior periods (1.4)    
Closing balance at 31 December 16.0 16.4 14.0
Current 0.0 0.0 0.0
Non-current 16.0 16.4 14.0
Interest and Penalties [Member]      
Income Tax Contingency [Line Items]      
Opening balance at January 1 2.0 0.6 0.3
Additions for tax positions of prior periods 0.6 1.4 0.4
Reductions due to lapsed statute of limitations     (0.1)
Reductions for tax positions of prior periods (0.2)    
Closing balance at 31 December 2.4 2.0 0.6
Current 0.0 0.0 0.0
Non-current 2.4 2.0 0.6
Unrecognized Tax Benefits [Member]      
Income Tax Contingency [Line Items]      
Opening balance at January 1 14.4 13.4 2.2
Additions for tax positions of prior periods 0.4 1.0 11.7
Reductions due to lapsed statute of limitations     (0.5)
Reductions for tax positions of prior periods (1.2)    
Closing balance at 31 December 13.6 14.4 13.4
Current 0.0 0.0 0.0
Non-current $ 13.6 $ 14.4 $ 13.4