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Income Taxes - Roll-forward of Unrecognized Tax Benefits and Associated Accrued Interest and Penalties (Detail) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2020
Dec. 31, 2019
Income Tax Contingency [Line Items]    
Opening balance at January 1 $ 16.4  
Net change for tax positions of prior periods (0.9)  
Closing balance at September 30 15.5  
Current 0.0  
Non-current 15.5 $ 16.4
Interest and Penalties [Member]    
Income Tax Contingency [Line Items]    
Opening balance at January 1 2.0  
Net change for tax positions of prior periods 0.2  
Closing balance at September 30 2.2  
Current 0.0  
Non-current 2.2  
Unrecognized Tax Benefits [Member]    
Income Tax Contingency [Line Items]    
Opening balance at January 1 14.4  
Net change for tax positions of prior periods (1.1)  
Closing balance at September 30 13.3  
Current 0.0  
Non-current $ 13.3