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Debt - Interest Expense - Convertible Senior Notes (Details) - Convertible Notes - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Cash Interest Expense    
Coupon interest expense $ 9,315 $ 8,901
Non-Cash Interest Expense    
Amortization of debt discount 10,686 9,490
Amortization of transaction costs 1,076 956
Total Interest Expense 21,077 $ 19,347
Estimated fair value $ 255,300  
Convertible Debt, Fair Value by Fair Value Hierarchy Level [Extensible List] Fair Value Inputs Level2 [Member]