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Debt - Interest Expense (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Long-term debt        
Amortization of debt issuance costs     $ 390 $ 308
Convertible debt | 3.00% Convertible senior notes due 2022        
Long-term debt        
Contractual interest expense $ 1,134 $ 1,134 3,375 3,375
Amortization of debt issuance costs 95 86 277 249
Amortization of debt discount 1,919 1,725 5,563 4,999
Total $ 3,148 $ 2,945 $ 9,215 $ 8,623
Effective interest rate of the liability component 11.00% 11.00% 11.00% 11.00%