Quarterly Operating Results (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Operating Results (Unaudited)
Revenues from Continuing Operations	$ 289,880	$ 307,783	$ 300,852	$ 243,483	$ 243,895	$ 250,370	$ 234,638	$ 194,921	$ 1,141,998	$ 923,824	$ 829,084
Operating income from Continuing Operations	35,730	50,832	55,886	14,295	24,640	34,368	36,622	3,568	156,743	99,198	79,010
Net income (loss)	$ 14,257	$ 33,643	$ 43,535	$ (3,496)	$ 5,249	$ 15,817	$ 20,009	$ 28,926	$ 87,939	$ 70,001	$ 49,557
Income available (loss attributable) to common stockholders per share - basic and diluted (in dollars per share)	$ 0.05	$ 0.14	$ 0.22	$ (0.04)	$ 0.01	$ 0.07	$ 0.09	$ 0.12	$ 0.37	$ 0.29	$ 0.14