Convertible Senior Notes - Conversion (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended	6 Months Ended		9 Months Ended		12 Months Ended
	May 18, 2020	May 31, 2020	Mar. 31, 2020	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	May 29, 2020	Mar. 31, 2018
Convertible Senior Notes
Net proceeds			$ 9,024	$ 27,678	$ 25,609	$ 47,568	$ 57,249	$ 65,259	$ 83,668	$ 76,175
5.5% Convertible Senior Notes
Convertible Senior Notes
Principal amount	$ 100,000							160	100,000			$ 100,000
Less initial purchasers' discount		$ (3,300)
Less other issuance costs		(900)							$ (1,700)
3.75% Convertible Senior Notes
Convertible Senior Notes
Principal amount	200,000	212,463						$ 212,463			$ 12,500
Less initial purchasers' discount	(6,400)	(6,374)
Less cost of related capped calls		(16,253)
Less other issuance costs	$ (600)	(617)
Net proceeds		$ 189,219