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, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016		Jun. 30, 2016	Mar. 31, 2016		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 223,957		$ 217,953		$ 208,049		$ 207,389	$ 204,109	$ 201,157		$ 197,981	$ 198,344		$ 857,348	$ 801,591	$ 744,012
Operating income	83,157		84,497		83,090		81,544	83,477	80,461		80,135	76,922		332,288	320,995	300,154
Net income	52,785	[1]	108,882	[1]	78,133	[1]	58,070	59,724	61,198	[2]	58,898	79,063	[2]	297,870	258,883	218,424
NET INCOME ATTRIBUTABLE TO THE TRUST	50,656	[1]	106,777	[1]	76,291	[1]	56,190	58,037	58,977	[2]	55,941	76,955	[2]	289,914	249,910	210,219
Net income available for common shareholders	$ 48,645	[1]	$ 106,600	[1]	$ 76,156	[1]	$ 56,055	$ 57,902	$ 58,841	[2]	$ 55,806	$ 76,820	[2]	$ 287,456	$ 249,369	$ 209,678
Earnings Per Common Share, Basic	$ 0.67	[1]	$ 1.47	[1]	$ 1.05	[1]	$ 0.78	$ 0.81	$ 0.82	[2]	$ 0.79	$ 1.10	[2]	$ 3.97	$ 3.51	$ 3.04
Net income available for common shareholders	$ 0.67	[1]	$ 1.47	[1]	$ 1.05	[1]	$ 0.78	$ 0.80	$ 0.82	[2]	$ 0.78	$ 1.10	[2]	$ 3.97	$ 3.50	$ 3.03

[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.