Selected Quarterly Financial Data (Unaudited) Summary Of Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended													12 Months Ended
	Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017		Jun. 30, 2017	Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 235,376		$ 229,753		$ 224,902		$ 225,405	$ 223,957	$ 217,953		$ 208,049	$ 207,389		$ 915,436	$ 857,348	$ 801,591
Operating income	83,338		90,342		89,896		86,145	83,157	84,497		83,090	81,544		349,721	332,288	320,995
Net income	56,382	[1]	64,180	[1]	65,533	[1]	62,931	52,785	108,882	[2]	78,133	58,070	[2]	249,026	297,870	258,883
NET INCOME ATTRIBUTABLE TO THE TRUST	54,507	[1]	62,558	[1]	63,595	[1]	61,247	50,656	106,777	[2]	76,291	56,190	[2]	241,907	289,914	249,910
Net income available for common shareholders	$ 52,496	[1]	$ 60,548	[1]	$ 61,584	[1]	$ 59,237	$ 48,645	$ 106,600	[2]	$ 76,156	$ 56,055	[2]	$ 233,865	$ 287,456	$ 249,369
Earnings Per Common Share, Basic	$ 0.71	[1]	$ 0.82	[1]	$ 0.84	[1]	$ 0.81	$ 0.67	$ 1.47	[2]	$ 1.05	$ 0.78	[2]	$ 3.18	$ 3.97	$ 3.51
Earnings Per Common Share, Diluted	$ 0.71	[1]	$ 0.82	[1]	$ 0.84	[1]	$ 0.81	$ 0.67	$ 1.47	[2]	$ 1.05	$ 0.78	[2]	$ 3.18	$ 3.97	$ 3.50

[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.