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DEBT - Convertible Senior Notes - Debt Issuance Costs (Details) - USD ($)
$ in Thousands
12 Months Ended
Aug. 07, 2017
Dec. 31, 2017
Debt    
Equity component, reduction to additional paid in capital   $ 65,361
Convertible senior notes | 2025 Notes    
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   1,925
Amortization of debt issuance costs   189
Amortization of debt discount   2,268
Total interest and amortization expense   $ 4,382