STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 57,376	$ 49,562	$ 164,580	$ 144,545
Other comprehensive income (loss), net of tax
Change in unrealized holding gains (losses) on available-for-sale securities	355	(521)	6,846	(3,004)
Comprehensive income	$ 57,731	$ 49,041	$ 171,426	$ 141,541