Selected Quarterly Financial Data (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 Y	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011 Q	Dec. 31, 2010	Dec. 31, 2009
Selected Quarterly Financial Data (Unaudited)
Revenues	$ 295,851	$ 318,689	$ 313,809	$ 281,984	$ 267,791	$ 281,198	$ 282,391	$ 254,108	$ 1,210,333	$ 1,085,488	$ 1,037,033
Net income (loss)	38,218	47,531	51,643	53,048	(92,956)	50,232	23,210	40,865	190,440	21,351	193,341
Net income (loss) available for common shareholders	30,748	40,061	44,173	45,578	(100,426)	42,762	15,740	33,395	160,560	(8,529)	163,461
Net income (loss) available for common shareholders per share (in dollars per share)	$ 0.25	$ 0.32	$ 0.36	$ 0.37	$ (0.81)	$ 0.35	$ 0.13	$ 0.27	$ 1.30	$ (0.07)	$ 1.51
Distributions per common share (in dollars per share)	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45
Period considered for computing sum of per common share amount (in quarters)									4
Loss on asset impairment					$ 163,681				$ 16,384	$ 163,681
Per share loss on asset impairment (in dollars per share)					1.33