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Liabilities - Convertible Senior Notes (Details) - Convertible Notes - USD ($)
$ in Thousands
3 Months Ended
Jan. 10, 2017
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Debt        
Principal amount $ 345,000 $ 345,000   $ 345,000
Interest rate (as a percent) 2.70%      
Proceeds received, net of transaction fees $ 335,800      
Unamortized debt discount   (49,537)   (52,336)
Unamortized transaction costs   (4,990)   (5,272)
Net carrying value   290,473   $ 287,392
Cash Interest Expense        
Coupon interest expense   2,329 $ 2,329  
Non-Cash Interest Expense        
Amortization of debt discount   2,799 2,598  
Amortization of transaction costs   282 261  
Total Interest Expense   5,410 $ 5,188  
Estimated fair value   $ 295,000    
Convertible Debt, Fair Value by Fair Value Hierarchy Level [Extensible List]   Fair Value Inputs Level2 [Member]