Quarterly Operating Results (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Quarterly Operating Results (Unaudited)
Revenues from Continuing Operations	$ 245,083	$ 212,334	$ 218,254	$ 159,058	$ 176,890	$ 152,475	$ 157,393	$ 137,786	$ 834,729	$ 624,544	$ 609,312
Operating income (loss) from Continuing Operations	28,626	3,671	27,979	(1,558)	14,977	8,943	12,933	2,592	58,718	39,445	4,083
Net income (loss)	$ 7,588	$ (16,553)	$ 38,929	$ 51,335	$ 35,672	$ 23,653	$ 308	$ (21,091)	$ 81,299	$ 38,542	$ (269,608)
Income available (loss attributable) to common stockholders per share - basic (in dollars per share)	$ 0.00	$ (0.20)	$ 0.27	$ 0.39	$ 0.28	$ 0.19	$ (0.05)	$ (0.27)	$ 0.45	$ 0.18	$ (4.17)
Income available (loss attributable) to common stockholders per share - diluted (in dollars per share)	$ 0.00	$ (0.20)	$ 0.26	$ 0.39	$ 0.28	$ 0.19	$ (0.05)	$ (0.27)	$ 0.45	$ 0.18	$ (4.17)