Condensed Consolidated Statements Of Equity And Perpetual Preferred Units (USD $)	Total	Common Shares Of Beneficial Interest	Additional Paid-In Capital	Distributions In Excess Of Net Income	Treasury Shares, At Cost	Accumulated Other Comprehensive Loss	Non-controlling Interests	Perpetual Preferred Units
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
Perpetual Preferred Units, Beginning Balance at Dec. 31, 2011								97,925,000
Net income	144,903,000			141,224,000			3,679,000	2,851,000
Other comprehensive income	23,000					23,000
Common shares issued	693,375,000	112,000	693,263,000
Net share awards	11,619,000		(2,477,000)		14,096,000
Employee stock purchase plan	1,331,000		613,000		718,000
Common share options exercised	13,273,000		1,840,000		11,433,000
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	(148,943,000)			(142,993,000)			(5,950,000)
Redemption of perpetual preferred units	(100,000,000)							(100,000,000)
Purchase of non-controlling interests	(10,241,000)		(13,285,000)				3,044,000
Ending Balance at Sep. 30, 2012	2,533,108,000	959,000	3,580,528,000	(692,235,000)	(425,756,000)	(660,000)	70,272,000
Perpetual Preferred Units, Ending Balance at Sep. 30, 2012								0
Beginning Balance at Dec. 31, 2012	2,626,708,000	962,000	3,587,505,000	(598,951,000)	(425,355,000)	(1,062,000)	63,609,000
Net income	211,172,000			206,368,000			4,804,000
Other comprehensive income	41,000					41,000
Common shares issued	40,098,000	6,000	40,092,000
Net share awards	13,606,000	(1,000)	987,000		12,620,000
Employee stock purchase plan	906,000		437,000		469,000
Common share options exercised	2,697,000		740,000		1,957,000
Change in classification of deferred compensation plan	(34,517,000)		(34,517,000)
Change in redemption of non-qualified share awards	(12,928,000)			(12,928,000)
Diversification of share awards within deferred compensation plan	353,000		221,000	132,000
Conversions of operating partnership units	0		71,000				(71,000)
Cash distributions declared to equity holders	(170,146,000)			(166,556,000)			(3,590,000)
Redemption of perpetual preferred units	0
Ending Balance at Sep. 30, 2013	$ 2,677,990,000	$ 967,000	$ 3,595,536,000	$ (571,935,000)	$ (410,309,000)	$ (1,021,000)	$ 64,752,000