STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 45,895	$ 38,833
Other comprehensive income, net of tax
Change in unrealized holding gains on available-for-sale securities	3,465	100
Comprehensive income	$ 49,360	$ 38,933