Condensed Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 11,254	$ 77,241	$ (33,668)	$ 108,502
Unrealized gain (loss) on short-term investment securities, net of deferred taxes of $33, ($6), $26 and ($14)	101	(11)	78	(24)
Interest rate derivative loss reclassified into earnings, net of taxes of $18, $31, $39 and $62	61	53	120	107
Other comprehensive income	162	42	198	83
Comprehensive income (loss)	$ 11,416	$ 77,283	$ (33,470)	$ 108,585