Debt - Convertible Senior Notes - Liability and Equity Components (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Liability component
Principal	$ 557,234	$ 677,234
Debt discount and issuance costs, net	(72,643)	(77,872)
Convertible senior notes
Liability component
Principal	192,500	$ 192,500
Convertible senior notes | 2025 Notes
Liability component
Principal	192,500
Debt discount and issuance costs, net	(66,740)
Net carrying amount	125,760
Equity component, net	$ 65,361