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 Income (Loss)	Noncontrolling 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	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
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
Perpetual Preferred Units, Ending Balance at Mar. 31, 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	64,433,000			63,476,000			957,000
Other comprehensive income	14,000					14,000
Common shares issued	9,366,000	1,000	9,365,000
Net share awards	4,741,000		(7,198,000)		11,939,000
Employee stock purchase plan	354,000		174,000		180,000
Common share options exercised	960,000		367,000		593,000
Conversions of operating partnership units	0		47,000				(47,000)
Cash distributions declared to equity holders	(56,553,000)			(55,356,000)			(1,197,000)
Redemption of perpetual preferred units	0
Other	0	(1,000)	1,000
Ending Balance at Mar. 31, 2013	$ 2,650,023,000	$ 962,000	$ 3,590,261,000	$ (590,831,000)	$ (412,643,000)	$ (1,048,000)	$ 63,322,000