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Debt - Interest Expense (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Long-term debt      
Amortization of debt issuance costs $ 433 $ 302 $ 107
Convertible debt | 3.00% Convertible senior notes due 2022      
Long-term debt      
Contractual interest expense 4,500 4,497  
Amortization of debt issuance costs 337 302  
Amortization of debt discount 6,755 6,065  
Total $ 11,592 $ 10,864  
Effective interest rate of the liability component 11.00% 11.00%