Condensed Consolidated Statements Of Equity (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
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 share 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)
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
Beginning balance at Dec. 31, 2013	2,760,181,000	967,000	3,596,069,000	(494,167,000)	(410,227,000)	(1,106,000)	68,645,000
Net income	41,073,000			40,036,000			1,037,000
Other comprehensive income	15,000					15,000
Net share awards	5,056,000		(5,287,000)		10,343,000
Employee share purchase plan	498,000		168,000		330,000
Common share options exercised	352,000		308,000		44,000
Change in classification of deferred compensation plan	606,000		606,000
Change in redemption value of non-qualified share awards	(11,352,000)			(11,352,000)
Diversification of share awards within deferred compensation plan	2,428,000		1,770,000	658,000
Cash distributions declared to equity holders	(59,750,000)			(58,496,000)			(1,254,000)
Other	(2,000)	(1,000)	(1,000)
Ending balance at Mar. 31, 2014	$ 2,739,105,000	$ 966,000	$ 3,593,633,000	$ (523,321,000)	$ (399,510,000)	$ (1,091,000)	$ 68,428,000