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	$ 505,624	$ 431,677	$ 384,677
Other comprehensive income (loss), net of tax:
Net changes in unrealized losses on available-for-sale investment securities	(2,126)	(22,628)	(10,381)
Foreign currency translation adjustments	12,753	(10,577)	(8,797)
Other comprehensive income (loss)	10,627	(33,205)	(19,178)
COMPREHENSIVE INCOME	$ 516,251	$ 398,472	$ 365,499