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Debt - Schedule of Debt Obligations (Parenthetical) (Detail) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Nov. 14, 2014
Aug. 06, 2014
Term Debt [Member]          
Debt Instrument [Line Items]          
Interest rate on debt 5.50% 5.10% 4.50%    
Senior Notes [Member]          
Debt Instrument [Line Items]          
Interest rate on debt 11.00% 11.00% 11.00%    
Revolving Credit Facility [Member]          
Debt Instrument [Line Items]          
Available capacity $ 300.0 $ 130.0 $ 130.0 $ 130.0 $ 130.0
Revolving Credit Facility [Member] | Letter of Credit [Member]          
Debt Instrument [Line Items]          
Available capacity $ 2.7 2.9 $ 7.7    
Revolving Credit Facility [Member] | Previously Reported [Member] | Letter of Credit [Member]          
Debt Instrument [Line Items]          
Available capacity   $ 8.4