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Debt - 5.00% Convertible Senior Notes Due 2021 (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Debt Instrument [Line Items]    
Principal amount of long-term debt $ 711,457 $ 416,466
Unamortized discount and deferred financing costs $ 34,628 23,826
5.00% Convertible Senior Notes due 2021    
Debt Instrument [Line Items]    
Debt instrument, interest rate 5.00%  
5.00% Convertible Senior Notes due 2021 | Carrying Value    
Debt Instrument [Line Items]    
Long-term debt $ 101,700  
Convertible Debt | 5.00% Convertible Senior Notes due 2021    
Debt Instrument [Line Items]    
Debt instrument, interest rate 5.00%  
Principal amount of long-term debt $ 115,000 $ 115,000
Unamortized discount and deferred financing costs $ 13,300