Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Revenue	$ 163,846	[1]	$ 159,030	[1]	$ 157,387	[1]	$ 157,150	[1]	$ 155,785	[1]	$ 157,232	[1]	$ 147,041	[1]	$ 145,733	[1]	$ 637,413	$ 605,791	$ 551,103
Operating Income	63,470		64,701		63,488		62,502		65,490		66,752		61,431		60,189		254,161	253,862	226,462
Net income	29,603		63,366	[2]	38,797		35,842		38,858		39,656		33,596		44,122		167,608	156,232	149,612
Net income attributable to the Trust	28,456		62,098	[2]	37,539		34,588		37,692		38,644		32,603		42,986		162,681	151,925	143,917
Net income available for common shareholders	28,321		61,962	[2]	37,404		34,453		37,557		38,508		32,468		42,851		162,140	151,384	143,376
Earnings per common share—basic	$ 0.43		$ 0.94	[2]	$ 0.57		$ 0.53		$ 0.58		$ 0.60		$ 0.51		$ 0.67		$ 2.47	$ 2.36	$ 2.29
Earnings per common share—diluted	$ 0.42		$ 0.94	[2]	$ 0.57		$ 0.53		$ 0.58		$ 0.60		$ 0.51		$ 0.67		$ 2.46	$ 2.35	$ 2.28
Revenue from discontinued operations	0		385		559		602		579		572		520		556		1,500	2,200	4,200
Discontinued operation - gain on sale of real estate			$ 23,900														$ 23,861	$ 0	$ 15,075

[1] Revenue has been reduced to reflect the results of discontinued operations. Revenue from discontinued operations, by quarter, is summarized as follows: FirstQuarter SecondQuarter ThirdQuarter FourthQuarter (In thousands)2013 revenue from discontinued operations$602 $559 $385 $—2012 revenue from discontinued operations$556 $520 $572 $579

[2] (2) Third quarter 2013 includes a $23.9 million gain on sale of two retail buildings as further discussed in Note 3.