Condensed Consolidated Statements of Comprehensive Income Statement - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 2,124	$ 9,063	$ 24,067	$ 45,398
Other comprehensive loss, net of tax
Net unrealized gain (loss) on short-term investments, net of tax	197	(35)	(487)	(35)
Other comprehensive loss, net of tax	197	(35)	(487)	(35)
Total comprehensive income	$ 2,321	$ 9,028	$ 23,580	$ 45,363