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Debt - Summary of Interest Expense (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Long-term debt      
Amortization of debt issuance costs $ 524 $ 433 $ 302
Convertible debt | 3.00% Convertible senior notes due 2022      
Long-term debt      
Contractual interest expense 4,500 4,500  
Amortization of debt issuance costs 375 337  
Amortization of debt discount 7,518 6,755  
Total $ 12,393 $ 11,592  
Effective interest rate of the liability component 11.00% 11.00%