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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, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Income Tax Contingency [Line Items]            
Opening balance at January 1 $ 3.9 $ 6.2 $ 13.0      
Reductions for tax positions of prior periods (0.7) 0.0 (4.0)      
Additions for tax positions of prior periods 0.0 0.4 0.1      
Additions for current year tax positions 0.0 1.2 0.2      
Reductions due to lapsed statutes of limitations (0.9) (3.9) (3.1)      
Closing balance at December 31 2.3 3.9 6.2      
Current       $ 0.0 $ 0.0 $ 0.0
Non-current       2.3 3.9 6.2
Unrecognized tax benefits 3.9 6.2 13.0 2.3 3.9 6.2
Unrecognized Tax Benefits [Member]            
Income Tax Contingency [Line Items]            
Opening balance at January 1 3.6 5.7 11.9      
Reductions for tax positions of prior periods (0.6) 0.0 (3.6)      
Additions for tax positions of prior periods 0.0 0.3 0.1      
Additions for current year tax positions 0.0 1.2 0.2      
Reductions due to lapsed statutes of limitations (0.8) (3.6) (2.9)      
Closing balance at December 31 2.2 3.6 5.7      
Current       0.0 0.0 0.0
Non-current       2.2 3.6 5.7
Unrecognized tax benefits 3.6 5.7 11.9 2.2 3.6 5.7
Interest and Penalties [Member]            
Income Tax Contingency [Line Items]            
Opening balance at January 1 0.3 0.5 1.1      
Reductions for tax positions of prior periods (0.1) 0.0 (0.4)      
Additions for tax positions of prior periods 0.0 0.1 0.0      
Additions for current year tax positions 0.0 0.0 0.0      
Reductions due to lapsed statutes of limitations (0.1) (0.3) (0.2)      
Closing balance at December 31 0.1 0.3 0.5      
Current       0.0 0.0 0.0
Non-current       0.1 0.3 0.5
Unrecognized tax benefits $ 0.3 $ 0.5 $ 1.1 $ 0.1 $ 0.3 $ 0.5