Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 420,606	$ 264,879	$ 317,220
Unrealized gain (loss) on interest rate derivative instruments, net of deferred taxes of $0, $0, and ($550)	0	0	(910)
Unrealized gain (loss) on short-term investment securities, net of deferred taxes of ($41), ($13), and $0	(82)	(23)	0
Interest rate derivative (gain) loss reclassified into earnings, net of taxes of $372, $130, and $50	(37)	224	82
Other comprehensive income (loss)	(119)	201	(828)
Comprehensive income	$ 420,487	$ 265,080	$ 316,392