CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income		$ 117,051	$ 106,935	$ 99,410
Other comprehensive income (loss)	[1]	(4,748)	127	(2,402)
Comprehensive income (loss)		$ 112,303	$ 107,062	$ 97,008

[1] Represents change in fair value of available-for-sale investments, net of income tax provision of $76 thousand for the year ended December 31, 2017 and a change in fair value of available-for-sale investments, net of income tax benefit of $1,560 thousand and $1,486 thousand for the years ended December 31, 2018 and 2016, respectively.