Convertible Senior Notes - Net proceeds (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Mar. 31, 2018
Convertible Senior Notes
Less cost of related capped call and common stock forward	$ (43,500)
Net proceeds	95,856
Convertible Senior Notes
Convertible Senior Notes
Interest rate (as a percent)		5.50%
Convertible Senior Notes
Principal amount	100,000
Less initial purchasers' discount	(3,250)
Less cost of related capped call and common stock forward	(43,500)
Less other issuance costs	(894)
Net proceeds	$ 52,356