CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 8,241	$ 7,807	$ 17,369	$ 26,423
Other comprehensive income:
Realized gains on short-term investments reclassified from accumulated other comprehensive income, net of tax of $0, $5, $0 and $3, respectively	0	(10)	0	(7)
Net change in unrealized gains on available-for-sale investments, net of tax of ($61), ($380), ($274) and ($2,182), respectively	135	832	346	3,350
Comprehensive income	$ 8,376	$ 8,629	$ 17,715	$ 29,766