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Debt - Schedule of Components of Long-term Debt (Details) - USD ($)
Jun. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]      
Other debt $ 20,500,000 $ 21,300,000  
Total 648,300,000 644,700,000  
Less – current maturities 10,200,000 10,800,000  
Total long-term debt $ 638,100,000 633,900,000  
5.50% convertible senior notes due March 1, 2021      
Debt Instrument [Line Items]      
Face value     $ 1,000
Senior Notes | 10.75% senior notes due April 15, 2022      
Debt Instrument [Line Items]      
Interest rate 10.75%    
Face value $ 440,000,000 440,000,000  
Unamortized discount and fees 9,200,000 10,400,000  
Senior notes $ 430,800,000 429,600,000  
Senior Notes | 5.50% convertible senior notes due March 1, 2021      
Debt Instrument [Line Items]      
Interest rate 5.50%   5.50%
Face value $ 213,500,000 213,500,000 $ 213,500,000.0
Unamortized discount and fees 23,300,000 27,200,000  
Senior notes 190,200,000 186,300,000  
Capital Lease Obligations      
Debt Instrument [Line Items]      
Capital leases $ 6,800,000 $ 7,500,000