Quarterly Operating Results (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Operating Results (Unaudited)
Revenues	$ 290,190	$ 303,909	$ 318,796	$ 280,743	$ 289,584	$ 303,304	$ 322,160	$ 274,292	$ 1,193,638	$ 1,189,340	$ 1,249,180
Operating income	34,948	13,072	62,703	30,282	37,068	48,846	63,422	21,079	141,005	170,415	180,436
Net income	$ 20,680	$ 17,082	$ 51,415	$ 63,827	$ 34,298	$ 39,427	$ 65,736	$ 1,216	$ 153,004	$ 140,677	$ 355,519
Income (loss) attributable to common stockholders per share - basic and diluted (in dollars per share)	$ 0.07	$ 0.05	$ 0.21	$ 0.27	$ 0.14	$ 0.16	$ 0.26	$ (0.02)	$ 0.59	$ 0.55	$ 1.62