Earnings Per Share (Schedule Of Earnings Per Share) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011		Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Earnings Per Share [Abstract]
Income from continuing operations										$ 131,554	$ 125,851	$ 101,325
Less: Preferred share dividends										(541)	(541)	(541)
Less: Income from continuing operations attributable to noncontrolling interests										(5,475)	(5,247)	(5,568)
Less: Earnings allocated to unvested shares										(705)	(572)	(510)
Income from continuing operations available for common shareholders										124,833	119,491	94,706
Results from discontinued operations attributable to the Trust										17,838	1,776	2,547
Gain on sale of real estate										0	410	0
Net income available for common shareholders, basic and diluted										142,671	121,677	97,253
Weighted average common shares outstanding-basic										62,438	61,182	59,704
Stock options										165	142	126
Weighted average common shares outstanding-diluted										62,603	61,324	59,830
Continuing operations										$ 2.00	$ 1.95	$ 1.59
Discontinued operations										$ 0.29	$ 0.03	$ 0.04
Gain on sale of real estate											$ 0.01
Earnings Per Share, Basic, Total	$ 0.48	$ 0.74	[1]	$ 0.55	$ 0.50	$ 0.53	$ 0.48	$ 0.50	$ 0.47	$ 2.29	$ 1.99	$ 1.63
Continuing operations										$ 1.99	$ 1.94	$ 1.59
Discontinued operations										$ 0.29	$ 0.03	$ 0.04
Gain on sale of real estate											$ 0.01
Earnings Per Share, Diluted, Total	$ 0.48	$ 0.74	[1]	$ 0.55	$ 0.50	$ 0.53	$ 0.48	$ 0.50	$ 0.47	$ 2.28	$ 1.98	$ 1.63
Income from continuing operations attributable to the Trust										$ 126,079	$ 120,604	$ 95,757

[1] Third quarter 2011 amounts include a $14.8 million gain on sale of our Feasterville Shopping Center as further discussed in Note 3.