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Debt - Interest Expense (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Sep. 30, 2017
Long-term debt          
Amortization of debt issuance costs     $ 280 $ 256  
Convertible debt | 3.00% Convertible senior notes due 2022          
Long-term debt          
Contractual interest expense $ 1,131 $ 1,131 2,241 2,241  
Amortization of debt issuance costs 103 93 202 182  
Amortization of debt discount 2,074 1,863 4,055 3,644  
Total $ 3,308 $ 3,087 $ 6,498 $ 6,067  
Effective interest rate of the liability component 11.00% 11.00% 11.00% 11.00% 11.00%