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Income Taxes - Roll-Forward of Unrecognized Tax Benefits (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Abstract]      
Opening balance at January 1 $ 2.2 $ 3.6 $ 5.7
Reductions for tax positions of prior periods 0.0 (0.6) 0.0
Additions for tax positions of prior periods 0.5 0.0 0.3
Additions for current year tax positions 0.0 0.0 1.2
Reductions due to lapsed statute of limitations (0.5) (0.8) (3.6)
Closing balance at 31 December $ 2.2 $ 2.2 $ 3.6