Condensed Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 56,076	$ (44,922)
Unrealized gain (loss) on short-term investment securities, net of deferred taxes of $39 and ($8)	130	(23)
Interest rate derivative loss reclassified into earnings, net of taxes of $27 and $21	47	58
Other comprehensive income	177	35
Comprehensive income (loss)	$ 56,253	$ (44,887)