Condensed Consolidated Statements Of Equity And Perpetual Preferred Units (USD $)	Common Shares Of Beneficial Interest [Member]	Additional Paid-In Capital [Member]	Distributions In Excess Of Net Income [Member]	Notes Receivable Secured By Common Shares [Member]	Treasury Shares, At Cost [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Noncontrolling Interests [Member]	Perpetual Preferred Units [Member]	Total
Perpetual preferred units, Beginning Balance at Dec. 31, 2009								$ 97,925,000
Beginning Balance at Dec. 31, 2009	770,000	2,525,656,000	(492,571,000)	(101,000)	(462,188,000)	(41,155,000)	78,602,000		1,609,013,000
Net income			6,069,000				542,000	5,250,000	6,611,000
Other comprehensive income (loss)						(147,000)			(147,000)
Common shares issued	29,000	134,588,000							134,617,000
Net share awards	4,000	8,919,000							8,923,000
Employee stock purchase plan		(150,000)			933,000				783,000
Common share options exercised	1,000	2,459,000							2,460,000
Conversions of operating partnership units	2,000	2,130,000					(2,132,000)		2,132,000
Cash distributions declared to perpetual preferred units								(5,250,000)
Cash distributions declared to equity holders			(93,544,000)				(3,863,000)		(97,407,000)
Other	(2,000)	4,000		101,000			10,000		113,000
Perpetual preferred units, Ending Balance at Sep. 30, 2010								97,925,000
Ending Balance at Sep. 30, 2010	804,000	2,673,606,000	(580,046,000)		(461,255,000)	(41,302,000)	73,159,000		1,664,966,000
Perpetual preferred units, Beginning Balance at Dec. 31, 2010								97,925,000	97,925,000
Beginning Balance at Dec. 31, 2010	824,000	2,775,625,000	(595,317,000)		(461,255,000)	(33,458,000)	70,954,000		1,757,373,000
Net income			2,529,000				2,118,000	5,250,000	4,647,000
Other comprehensive income (loss)						33,659,000			33,659,000
Common shares issued	11,000	69,852,000							69,863,000
Net share awards	4,000	9,597,000			703,000				10,304,000
Employee stock purchase plan		482,000			1,335,000				1,817,000
Common share options exercised	1,000	4,990,000			6,973,000				11,964,000
Conversions of operating partnership units	1,000	591,000					(592,000)		592,000
Cash distributions declared to perpetual preferred units								(5,250,000)
Cash distributions declared to equity holders			(108,109,000)				(3,669,000)		(111,778,000)
Other	(2,000)	2,000
Perpetual preferred units, Ending Balance at Sep. 30, 2011								97,925,000	97,925,000
Ending Balance at Sep. 30, 2011	$ 839,000	$ 2,861,139,000	$ (700,897,000)		$ (452,244,000)	$ 201,000	$ 68,811,000		$ 1,777,849,000