Earnings Per Share (Schedule Of Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended													12 Months Ended
	Dec. 31, 2018	[1]	Sep. 30, 2018	[1]	Jun. 30, 2018	[1]	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	[2]	Jun. 30, 2017	Mar. 31, 2017	[2]	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Income from continuing operations														$ 237,111	$ 219,948	$ 226,425
Less: Preferred share dividends														(8,042)	(2,458)	(541)
Less: Income from continuing operations attributable to noncontrolling interests														(7,119)	(7,666)	(7,648)
Less: Earnings allocated to unvested shares														(930)	(942)	(702)
Income from continuing operations available for common shareholders														221,020	208,882	217,534
Gain on sale of real estate and change in control of interests, net														(11,915)	(77,632)	(31,133)
Net income available for common shareholders, basic and diluted														$ 232,935	$ 286,514	$ 248,667
Weighted average common shares outstanding—basic														73,274	72,117	70,877
Stock options														28	116	172
Weighted average common shares outstanding—diluted														73,302	72,233	71,049
Earnings Per Common Share, Basic	$ 0.71		$ 0.82		$ 0.84		$ 0.81	$ 0.67	$ 1.47		$ 1.05	$ 0.78		$ 3.18	$ 3.97	$ 3.51
Earnings Per Common Share, Diluted	$ 0.71		$ 0.82		$ 0.84		$ 0.81	$ 0.67	$ 1.47		$ 1.05	$ 0.78		$ 3.18	$ 3.97	$ 3.50
Income from continuing operations attributable to the Trust														$ 229,992	$ 212,282	$ 218,777

[1] First and second quarter 2018 include net gains of $3.3 million and $4.0 million, respectively, related to condominiums sold at our Assembly Row and Pike & Rose properties. Third quarter 2018 includes a $3.1 million gain on sale of real estate from our residential building at our Chelsea Commons property. Fourth quarter 2018 includes a $1.6 million gain on sale of real estate from our Atlantic Plaza property. All of these transactions are further discussed in Note 3.

[2] Second quarter 2017 includes a $15.4 million gain related to the sale of three ground lease parcels at our Assembly Row property in Somerville, Massachusetts. Third quarter 2017 includes a $50.8 million gain on sale of real estate from our 150 Post Street and North Lake Commons properties. Fourth quarter 2017 includes a $6.5 million gain related to the sale of a parcel of land at our Bethesda Row property. Additionally, second, third, and fourth quarter 2017 include net percentage-of-completion gains of $3.3 million, $0.6 million, and $1.5 million, respectively, related to condominiums under binding contract at our Assembly Row property. All of these transactions are further discussed in Note 3. Fourth quarter 2017 includes a $12.3 million early extinguishment of debt charge.