Condensed Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 83,464	$ 97,480	$ 254,041	$ 63,812
Unrealized gain (loss) on short-term investment securities, net of deferred taxes of ($20), ($5), $47, and $21	(68)	(7)	161	70
Interest rate derivative loss reclassified into earnings, net of taxes of $17, $27, $63, and $66	57	52	157	172
Other comprehensive income (loss)	(11)	45	318	242
Comprehensive income	$ 83,453	$ 97,525	$ 254,359	$ 64,054