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Liabilities - Convertible Senior Notes (Details) - Convertible Notes - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Jan. 10, 2017
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Debt            
Principal amount $ 345,000 $ 345,000   $ 345,000   $ 345,000
Interest rate (as a percent) 2.70% 2.70%   2.70%    
Proceeds received, net of transaction fees $ 335,800          
Unamortized debt discount   $ (43,780)   $ (43,780)   (52,336)
Unamortized transaction costs   (4,410)   (4,410)   (5,272)
Net carrying value   296,810   296,810   $ 287,392
Cash Interest Expense            
Coupon interest expense   2,329 $ 2,329 6,986 $ 6,986  
Non-Cash Interest Expense            
Amortization of debt discount   2,906 2,697 8,556 7,940  
Amortization of transaction costs   293 271 862 799  
Total Interest Expense   $ 5,528 $ 5,297 $ 16,404 $ 15,725  
Convertible Debt, Fair Value by Fair Value Hierarchy Level   us-gaap:FairValueInputsLevel2Member   us-gaap:FairValueInputsLevel2Member    
Estimated fair value   $ 309,200   $ 309,200