Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 335,255	$ 155,749	$ 415,522
Unrealized gain (loss) on short-term investment securities and cash and cash equivalents, net of deferred taxes of $38, $44, and ($41)	167	30	(82)
Interest rate derivative loss reclassified into earnings, net of taxes of $76, $75, and $372	239	241	(37)
Other comprehensive income (loss)	406	271	(119)
Comprehensive income	$ 335,661	$ 156,020	$ 415,403