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Senior Convertible Notes (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Senior Convertibles Notes [Line Items]          
Senior Convertible Notes, net of unamortized discount and deferred financing costs $ 154,883   $ 154,883   $ 147,894
1.50% Senior Convertible Notes Due 2021 [Member]          
Senior Convertibles Notes [Line Items]          
Senior Convertible Notes, principal amount $ 172,500   $ 172,500   172,500
Senior Convertible Notes, interest rate, stated percentage 1.50%   1.50%    
Senior Convertible Notes, interest expense $ 2,800 $ 2,600 $ 8,200 $ 7,800  
Senior Convertible Notes, unamortized debt discount (16,012)   (16,012)   (22,313)
Senior Convertible Notes, unamortized deferred financing costs (1,605)   (1,605)   (2,293)
Senior Convertible Notes, net of unamortized discount and deferred financing costs $ 154,883   $ 154,883   $ 147,894