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, 2017	[1]	Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	[2]	Jun. 30, 2016	Mar. 31, 2016	[2]	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Income from continuing operations														$ 219,948	$ 226,425	$ 190,094
Less: Preferred share dividends														(2,458)	(541)	(541)
Less: Income from continuing operations attributable to noncontrolling interests														(7,666)	(7,648)	(8,205)
Less: Earnings allocated to unvested shares														(942)	(702)	(797)
Income from continuing operations available for common shareholders														208,882	217,534	180,551
Gain on sale of real estate and change in control of interests, net														(77,632)	(31,133)	(28,330)
Net income available for common shareholders, basic and diluted														$ 286,514	$ 248,667	$ 208,881
Weighted average common shares outstanding—basic														72,117	70,877	68,797
Stock options														116	172	184
Weighted average common shares outstanding—diluted														72,233	71,049	68,981
Earnings Per Common Share, Basic	$ 0.67		$ 1.47		$ 1.05		$ 0.78	$ 0.81	$ 0.82		$ 0.79	$ 1.10		$ 3.97	$ 3.51	$ 3.04
Earnings Per Common Share, Diluted	$ 0.67		$ 1.47		$ 1.05		$ 0.78	$ 0.80	$ 0.82		$ 0.78	$ 1.10		$ 3.97	$ 3.50	$ 3.03
Income from continuing operations attributable to the Trust														$ 212,282	$ 218,777	$ 181,889

[1] 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 as further discussed in Note 5.

[2] First quarter 2016 includes a $25.7 million gain on change in control of interests from our Clarion Partners acquisition as further discussed in Note 3. Third quarter 2016 includes a $4.9 million gain on sale from the reversal of our warranty reserve on condominiums sold at Santana Row as further discussed in Note 7.