STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 79,706	$ 58,527	$ 145,464	$ 106,403
Other comprehensive (loss) income, net of tax
Change in unrealized holding (losses) gains on available-for-sale securities	(551)	1,092	(5,978)	2,613
Comprehensive income	$ 79,155	$ 59,619	$ 139,486	$ 109,016