Condensed Consolidated Statements Of Equity And Perpetual Preferred Units (USD $)	Total	Common Shares Of Beneficial Interest [Member]	Additional Paid-In Capital [Member]	Distributions In Excess Of Net Income [Member]	Treasury Shares, At Cost [Member]	Accumulated Other Comprehensive Loss [Member]	Noncontrolling Interests [Member]	Perpetual Preferred Units [Member]
Perpetual preferred units, Beginning Balance at Dec. 31, 2010								$ 97,925,000
Beginning Balance at Dec. 31, 2010	1,757,373,000	824,000	2,775,625,000	(595,317,000)	(461,255,000)	(33,458,000)	70,954,000
Net income	7,851,000			7,286,000			565,000	1,750,000
Other comprehensive income	1,954,000					1,954,000
Common shares issued	3,795,000	1,000	3,794,000
Net share awards	3,551,000	4,000	3,547,000
Employee stock purchase plan	821,000		33,000		788,000
Share awards placed into deferred plans	0	(2,000)	2,000
Common share options exercised	616,000		616,000
Conversions of operating partnership units	0		4,000				(4,000)
Cash distributions declared to perpetual preferred units								(1,750,000)
Cash distributions declared to equity holders	(36,940,000)			(35,709,000)			(1,231,000)
Redemption of perpetual preferred units	0
Perpetual preferred units, Ending Balance at Mar. 31, 2011								97,925,000
Ending Balance at Mar. 31, 2011	1,739,021,000	827,000	2,783,621,000	(623,740,000)	(460,467,000)	(31,504,000)	70,284,000
Perpetual preferred units, Beginning Balance at Dec. 31, 2011	97,925,000							97,925,000
Beginning Balance at Dec. 31, 2011	1,827,768,000	845,000	2,901,024,000	(690,466,000)	(452,003,000)	(683,000)	69,051,000
Net income	90,253,000			88,758,000			1,495,000	2,851,000
Other comprehensive income	8,000					8,000
Common shares issued	435,911,000	73,000	435,838,000
Net share awards	4,119,000		(9,110,000)		13,229,000
Employee stock purchase plan	318,000		148,000		170,000
Common share options exercised	1,899,000		510,000		1,389,000
Conversions of operating partnership units	0	2,000	(450,000)				448,000
Cash distributions declared to perpetual preferred units								(776,000)
Cash distributions declared to equity holders	(48,745,000)			(46,366,000)			(2,379,000)
Redemption of perpetual preferred units	100,000,000							(100,000,000)
Other	0	(1,000)	1,000
Perpetual preferred units, Ending Balance at Mar. 31, 2012	0							0
Ending Balance at Mar. 31, 2012	$ 2,311,531,000	$ 919,000	$ 3,327,961,000	$ (648,074,000)	$ (437,215,000)	$ (675,000)	$ 68,615,000