Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 264,879	$ 317,220	$ 225,464
Unrealized gain (loss) on interest rate derivative instruments, net of deferred taxes of $0, ($550) and ($423)	0	(910)	(718)
Unrealized gain (loss) on short-term investment securities, net of deferred taxes of ($13), $0 and $0	(23)	0	0
Interest rate derivative losses reclassified into earnings, net of taxes of $130, $50, and $0	224	82	0
Other comprehensive income (loss)	201	(828)	(718)
Comprehensive income	$ 265,080	$ 316,392	$ 224,746