CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 15,994	$ 15,027	$ 44,933	$ 46,946
Short term investments:
Unrealized gain (loss), net of income tax expense (benefit) of $2, ($6), ($30) and $1, respectively	5	(10)	(111)	4
Comprehensive income	$ 15,999	$ 15,017	$ 44,822	$ 46,950