STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 47,876	$ 45,895
Other comprehensive income, net of tax
Change in unrealized holding gains on available-for-sale securities	1,521	3,465
Comprehensive income	$ 49,397	$ 49,360