Debt - Convertible Senior Notes - Liability and Equity Components (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Liability component
Outstanding principal balance	$ 447,234	$ 447,234
Debt discount and issuance costs, net	(62,490)	(64,379)
Convertible senior notes | 2025 Notes
Liability component
Outstanding principal balance	192,500	192,500
Debt discount and issuance costs, net	(60,032)	(61,766)
Net carrying amount	132,468	130,734
Equity component, net	$ 65,361	$ 65,361