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Debt - Components of Debt (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Jan. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]      
1.75% convertible senior notes due 2025 $ 450,000    
Debt issuance costs, unamortized (8,440)    
Discount on debt (125,002)    
Long-term debt, net 316,558   $ 55,567
Convertible Notes Payable      
Debt Instrument [Line Items]      
Discount on debt $ (125,000)    
Convertible Notes Payable | 1.75% Convertible Senior Note Due 2025      
Debt Instrument [Line Items]      
Debt issuance costs, unamortized   $ (14,200)  
Proceeds from issuance of 1.75% convertible senior notes due 2025 1.75%