Quarterly Operating Results (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Operating Results (Unaudited)
Revenues	$ 289,584	$ 303,304	$ 322,160	$ 274,292	$ 300,933	$ 324,595	$ 339,267	$ 284,385	$ 1,189,340	$ 1,249,180	$ 1,141,998
Operating income	37,068	48,846	63,422	21,079	41,141	52,274	69,353	17,668	170,415	180,436	156,743
Net income	$ 34,298	$ 39,427	$ 65,736	$ 1,216	$ 237,575	$ 63,084	$ 53,657	$ 1,203	$ 140,677	$ 355,519	$ 87,939
Income (loss) attributable to common stockholders per share - basic and diluted (in dollars per share)	$ 0.14	$ 0.16	$ 0.26	$ (0.02)	$ 1.12	$ 0.28	$ 0.23	$ (0.02)	$ 0.55	$ 1.62	$ 0.37