STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 80,446	$ 58,543	$ 225,910	$ 164,946
Other comprehensive (loss) income, net of tax
Change in unrealized holding (losses) gains on available-for-sale securities	(492)	(167)	(6,470)	2,446
Comprehensive income	$ 79,954	$ 58,376	$ 219,440	$ 167,392