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Convertible Senior Notes - Notes Payable (Details) - USD ($)
Sep. 30, 2018
May 22, 2018
Dec. 31, 2017
Aug. 31, 2014
Notes Payable, Current and Noncurrent [Abstract]        
Carrying value of equity component of 2023 Notes $ 0   $ 18,859,000  
Convertible Notes | 2019 Convertible Senior Notes        
Notes Payable, Current and Noncurrent [Abstract]        
Principal amount of 2019 Notes outstanding 223,215,000   245,000,000 $ 245,000,000.0
Unamortized discount (including unamortized debt issuance cost) (10,071,000)   (20,471,000)  
Total current portion of notes payable 213,144,000   224,529,000  
Convertible Notes | 2023 Convertible Senior Notes        
Notes Payable, Current and Noncurrent [Abstract]        
Principal amount of 2019 Notes outstanding 750,000,000 $ 750,000,000 0  
Unamortized discount (including unamortized debt issuance cost) (147,161,000)   0  
Total long-term portion of notes payable 602,839,000   0  
Carrying value of equity component of 2023 Notes 143,986,000   0  
Fair value of convertible senior notes outstanding (Level 2) $ 1,729,179,000   $ 446,360,000