Condensed Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ (44,922)	$ 31,261
Unrealized gain (loss) on short-term investment securities, net of deferred taxes of ($8) and ($8)	(23)	(13)
Interest rate derivative loss reclassified into earnings, net of taxes of $21 and $31	58	53
Other comprehensive income (loss)	35	40
Comprehensive income (loss)	$ (44,887)	$ 31,301