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Debt - Convertible Senior Note (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Principal amount of long-term debt $ 415,000 $ 415,000
Unamortized discount and deferred financing costs $ 25,402 30,188
5.00% Convertible Senior Notes due 2021    
Debt Instrument [Line Items]    
Interest rate 5.00%  
5.00% Convertible Senior Notes due 2021 | Carrying Value    
Debt Instrument [Line Items]    
Long-term debt, fair value $ 99,145 95,486
Convertible Debt | 5.00% Convertible Senior Notes due 2021    
Debt Instrument [Line Items]    
Interest rate 5.00%  
Principal amount of long-term debt $ 115,000 $ 115,000
Unamortized discount and deferred financing costs $ 15,900