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Debt - Interest Expense (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Sep. 30, 2017
Long-term debt      
Amortization of debt issuance costs $ 139 $ 126  
Convertible debt | 3.00% Convertible senior notes due 2022      
Long-term debt      
Contractual interest expense 1,110 1,110  
Amortization of debt issuance costs 99 89  
Amortization of debt discount 1,982 1,780  
Total $ 3,191 $ 2,979  
Effective interest rate of the liability component 11.00%   11.00%