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Debt - Convertible Senior Notes - Interest and Amortization Expense (Details) - Convertible senior notes - 2025 Notes - USD ($)
$ in Thousands
3 Months Ended
Aug. 07, 2017
Mar. 31, 2018
Debt    
Debt issuance costs $ 5,400  
Equity component, reduction to additional paid in capital 1,900  
Liability component, reduction to carrying value of debt $ 3,500  
Interest expense    
Contractual interest expense   $ 1,190
Amortization of debt issuance costs   123
Amortization of debt discount   1,479
Total interest and amortization expense   $ 2,792