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Convertible Senior Notes - Notes Payable (Details) - Convertible Notes - 2023 Convertible Senior Notes - USD ($)
Sep. 30, 2021
Dec. 31, 2020
May 31, 2018
Notes Payable, Current and Noncurrent [Abstract]      
Principal amount of the 2023 Notes outstanding $ 343,301,000 $ 495,280,000 $ 750,000,000
Unamortized discount (including unamortized debt issuance cost) (26,412,000) (52,987,000)  
Total long-term portion of notes payable 316,889,000 442,293,000  
Carrying value of equity component of the 2023 Notes 24,124,000 48,397,000  
Fair value of the 2023 Notes outstanding (Level 2) $ 343,888,000 $ 466,053,000