Condensed Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 60,190	$ 81,382	$ 170,268	$ 216,386
Unrealized gain (loss) on short-term investment securities, net of deferred taxes of $7, $3, ($6) and $3	13	4	(11)	4
Interest rate derivative losses reclassified into earnings, net of taxes of $31, $32, $92 and $97	53	56	160	170
Other comprehensive income (loss)	66	60	149	174
Comprehensive income	$ 60,256	$ 81,442	$ 170,417	$ 216,560