Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 155,749	$ 415,522	$ 263,483
Unrealized gain (loss) on short-term investment securities, net of deferred taxes of $44, ($41), and ($13)	30	(82)	(23)
Interest rate derivative loss reclassified into earnings, net of taxes of $75, $372 and $130	241	(37)	224
Other comprehensive income (loss)	271	(119)	201
Comprehensive income	$ 156,020	$ 415,403	$ 263,684