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Income Taxes - Roll-Forward of Unrecognized Tax Benefits and Associated Accrued Interest and Penalties (Detail) - USD ($)
$ in Millions
 6 Months Ended
Jun. 30, 2017
Dec. 31, 2016
Income Tax Contingency [Line Items]    
Opening balance at January 1 $ 2.3  
Adjustment for prior period tax positions 0.0  
Closing balance at June 30 2.3  
Current 0.0  
Non-current 2.3 $ 2.3
Unrecognized Tax Benefits [Member]    
Income Tax Contingency [Line Items]    
Opening balance at January 1 2.2  
Adjustment for prior period tax positions 0.0  
Closing balance at June 30 2.2  
Current 0.0  
Non-current 2.2  
Interest and Penalties [Member]    
Income Tax Contingency [Line Items]    
Opening balance at January 1 0.1  
Adjustment for prior period tax positions 0.0  
Closing balance at June 30 0.1  
Current 0.0  
Non-current $ 0.1