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Debt - Schedule of Debt Obligations (Parenthetical) (Detail) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Nov. 14, 2014
Term Debt [Member]      
Debt Instrument [Line Items]      
Interest rate on debt 5.40% 5.10%  
Senior Notes [Member]      
Debt Instrument [Line Items]      
Interest rate on debt 11.00% 11.00%  
Revolving Credit Facility [Member]      
Debt Instrument [Line Items]      
Available capacity $ 300.0 $ 130.0 $ 130.0
Revolving Credit Facility [Member] | Letter of Credit [Member]      
Debt Instrument [Line Items]      
Available capacity $ 2.7 $ 2.9