CONSOLIDATED STATEMENT OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 231,418	$ 189,682	$ 144,020
Net change in foreign currency translation adjustment	1,741	(670)	(154)
Change in unrealized gains (losses) on investments, net of tax	(232)	712	(686)
Other comprehensive income (loss)	1,509	42	(840)
Comprehensive income	$ 232,927	$ 189,724	$ 143,180