Condensed Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 31,935	$ 61,920
Unrealized gain (loss) on short-term investment securities, net of deferred taxes of ($8) and $0	(13)	0
Interest rate derivative losses reclassified into earnings, net of taxes of $31 and $33	53	57
Other comprehensive income (loss)	40	57
Comprehensive income	$ 31,975	$ 61,977