Noncontrolling Interests (Details) (USD $) In Thousands, 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
Noncontrolling Interests in the Operating Partnership [Roll Forward]
Beginning noncontrolling interests in the Operating Partnership				$ 120,838				$ 129,769	$ 120,838	$ 129,769
Adjustment of noncontrolling interests in the Operating Partnership to fair value									(3,955)	(2,721)	27,717
Conversion of Common Units to Common Stock									(1,906)	(3,061)	(5,591)
Net income attributable to noncontrolling interests in the Operating Partnership	595	366	623	507	501	366	1,933	520	2,091	3,320	3,197
Distributions to noncontrolling interests in the Operating Partnership									(6,413)	(6,469)	(6,832)
Total noncontrolling interests in the Operating Partnership	110,655				120,838				110,655	120,838	129,769
Net Income Available for Common Stockholders and Transfers From Noncontrolling Interests in the Operating Partnership
Net income available for common stockholders	11,427	7,002	10,112	10,136	9,017	6,878	36,224	9,671	38,677	61,790	51,778
Increase in additional paid in capital from conversion of Common Units to Common Stock									1,906	3,060	5,589
Change from net income available for common stockholders and transfers from noncontrolling interests									$ 40,583	$ 64,850	$ 57,367
Noncontrolling Interests in Consolidated Affiliates
Consolidated joint venture, partner's interest (in hundredths)	50.00%								50.00%