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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, 2021
Dec. 31, 2020
Income Tax Contingency [Line Items]    
Opening balance at January 1 $ 16.0  
Net change for tax positions of prior periods 0.3  
Closing balance at 30 June 16.3  
Current 0.0  
Non-current 16.3 $ 16.0
Interest and Penalties [Member]    
Income Tax Contingency [Line Items]    
Opening balance at January 1 2.4  
Net change for tax positions of prior periods 0.4  
Closing balance at 30 June 2.8  
Current 0.0  
Non-current 2.8  
Unrecognized Tax Benefits [Member]    
Income Tax Contingency [Line Items]    
Opening balance at January 1 13.6  
Net change for tax positions of prior periods (0.1)  
Closing balance at 30 June 13.5  
Current 0.0  
Non-current $ 13.5