STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 61,309	$ 56,150	$ 107,204	$ 94,983
Other comprehensive income (loss), net of tax
Change in unrealized holding gains (losses) on available-for-sale securities	3,026	(2,583)	6,491	(2,483)
Comprehensive income	$ 64,335	$ 53,567	$ 113,695	$ 92,500