Quarterly Operating Results (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Operating Results (Unaudited)
Revenues	$ 300,933	$ 324,595	$ 339,267	$ 284,385	$ 289,880	$ 307,783	$ 300,852	$ 243,483	$ 1,249,180	$ 1,141,998	$ 923,824
Operating income	41,141	52,274	69,353	17,668	35,730	50,832	55,886	14,295	180,436	156,743	99,198
Net income (loss)	$ 237,575	$ 63,084	$ 53,657	$ 1,203	$ 14,257	$ 33,643	$ 43,535	$ (3,496)	$ 355,519	$ 87,939	$ 70,001
Income (loss) attributable to common stockholders per share - basic and diluted (in dollars per share)	$ 1.12	$ 0.28	$ 0.23	$ (0.02)	$ 0.05	$ 0.14	$ 0.22	$ (0.04)	$ 1.62	$ 0.37	$ 0.29