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Convertible Notes and Credit Facility - Components of Notes (Details) - Convertible Debt - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Jun. 30, 2018
Mar. 31, 2018
Feb. 28, 2014
Debt Instrument [Line Items]          
Net carrying value of convertible notes $ 49,853 $ 172,058      
2014 Notes          
Debt Instrument [Line Items]          
Interest rate on notes 2.75%       2.75%
Principal amount $ 51,250 51,250      
Unamortized debt discount (1,182) (1,232)     $ (6,000)
Unamortized debt issuance cost (215) (224)      
Net carrying value of convertible notes $ 49,853 49,794      
2018 Notes          
Debt Instrument [Line Items]          
Interest rate on notes 2.75%   2.75% 2.75%  
Principal amount $ 0 149,999   $ 150,000  
Premium accretion 0 3,755      
Unamortized debt discount 0 (29,558)      
Unamortized debt issuance cost 0 (1,932)      
Net carrying value of convertible notes $ 0 $ 122,264