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Convertible Senior Notes - Notes Payable (Details) - 2019 Convertible Senior Notes - USD ($)
Mar. 31, 2018
Dec. 31, 2017
Notes Payable, Current and Noncurrent [Abstract]    
Principal amount outstanding $ 245,000,000 $ 245,000,000
Unamortized discount (including unamortized debt issuance cost) (17,453,000) (20,471,000)
Total current portion of notes payable $ 227,547,000 $ 224,529,000