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Income Taxes - Roll-forward of Unrecognized Tax Benefits and Associated Accrued Interest and Penalties (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Income Tax Contingency [Line Items]    
Opening balance at January 1 $ 14.0  
Additions for tax positions of prior periods 0.2  
Closing balance at March 31 14.2 $ 14.0
Current 0.0  
Non-current 14.2 14.0
Unrecognized Tax Benefits [Member]    
Income Tax Contingency [Line Items]    
Opening balance at January 1 13.4  
Additions for tax positions of prior periods 0.0  
Closing balance at March 31 13.4 13.4
Current 0.0  
Non-current 13.4  
Interest and Penalties [Member]    
Income Tax Contingency [Line Items]    
Opening balance at January 1 0.6  
Additions for tax positions of prior periods 0.2  
Closing balance at March 31 0.8 $ 0.6
Current 0.0  
Non-current $ 0.8