CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 57,872	$ 35,469	$ 26,143
Short term investments:
Unrealized loss, net of income tax benefit of $63, $11 and $8, respectively	(116)	(21)	(16)
Comprehensive income	$ 57,756	$ 35,448	$ 26,127