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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, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Tax Contingency [Line Items]      
Opening balance at January 1 $ 2.5 $ 2.3 $ 3.9
Reductions for tax positions of prior periods 0.0 0.0 0.7
Additions for tax positions of prior periods 12.1 0.7 0.0
Additions for current year tax positions 0.0 0.0 0.0
Reductions due to lapsed statute of limitations (0.6) (0.5) (0.9)
Closing balance at 31 December 14.0 2.5 2.3
Current 0.0 0.0 0.0
Non-current 14.0 2.5 2.3
Interest and Penalties [Member]      
Income Tax Contingency [Line Items]      
Opening balance at January 1 0.3 0.1 0.3
Reductions for tax positions of prior periods 0.0 0.0 0.1
Additions for tax positions of prior periods 0.4 0.2 0.0
Additions for current year tax positions 0.0 0.0 0.0
Reductions due to lapsed statute of limitations (0.1) 0.0 (0.1)
Closing balance at 31 December 0.6 0.3 0.1
Current 0.0 0.0 0.0
Non-current 0.6 0.3 0.1
Unrecognized Tax Benefits [Member]      
Income Tax Contingency [Line Items]      
Opening balance at January 1 2.2 2.2 3.6
Reductions for tax positions of prior periods 0.0 0.0 0.6
Additions for tax positions of prior periods 11.7 0.5 0.0
Additions for current year tax positions 0.0 0.0 0.0
Reductions due to lapsed statute of limitations (0.5) (0.5) (0.8)
Closing balance at 31 December 13.4 2.2 2.2
Current 0.0 0.0 0.0
Non-current $ 13.4 $ 2.2 $ 2.2