Condensed Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 73,084	$ 76,704	$ 135,004	$ 145,706
Unrealized gain (loss) on interest rate derivative instruments, net of deferred taxes of $0, $749, $0 and ($191)	0	1,238	0	(356)
Losses reclassified from other comprehensive income	88	0	178	0
Other comprehensive income (loss)	88	1,238	178	(356)
Comprehensive income	$ 73,172	$ 77,942	$ 135,182	$ 145,350