Condensed Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 78,143	$ 73,084	$ 110,078	$ 135,004
Unrealized gain (loss) on short-term investment securities, net of deferred taxes of ($6), $0, ($14) and $0	(11)	0	(24)	0
Interest rate derivative losses reclassified into earnings, net of taxes of $31, $32, $62 and $65	53	56	107	113
Other comprehensive income (loss)	42	56	83	113
Comprehensive income	$ 78,185	$ 73,140	$ 110,161	$ 135,117