Convertible Senior Notes - Conversion (Details) - USD ($) $ in Thousands		1 Months Ended	9 Months Ended
	Mar. 08, 2018	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018
Convertible Senior Notes
Less cost of related capped call and common stock forward				$ (43,500)
$40M Convertible Senior Note
Convertible Senior Notes
Less initial purchasers' discount		$ (8,000)
$100M Convertible Senior Notes
Convertible Senior Notes
Less initial purchasers' discount			$ (3,250)
Less initial purchasers' discount	$ (3,300)
Less cost of related capped call and common stock forward			(43,500)
Less other issuance costs	$ (900)	(1,700)
Less other issuance costs		$ (894)	$ (894)