Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income	$ 317,220	$ 225,464	$ 176,918
Unrealized gain (loss) on interest rate derivative instruments, net of deferred tax expense (benefit) of ($500), ($423) and $0	(828)	(718)	0
Other comprehensive income (loss)	(828)	(718)	0
Comprehensive income	$ 316,392	$ 224,746	$ 176,918