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Real Estate (Narrative) (Details) $ in Thousands				1 Months Ended	2 Months Ended	3 Months Ended				6 Months Ended
	Jun. 25, 2020 USD ($) ft²	Feb. 20, 2020 USD ($) ft² Buildings	Jan. 28, 2020 USD ($) ft² Buildings	Jan. 27, 2020 USD ($)	Feb. 19, 2020 USD ($)	Jun. 30, 2020 USD ($)		Jun. 24, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)		Jun. 24, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 26, 2020 USD ($) ft²	Jun. 01, 2020 ft²	Mar. 26, 2020 ft²
Real Estate Properties [Line Items]
Proceeds from sales of real estate										$ 506,337			$ 60,398
Gains on sales of real estate						$ 203,767			$ 1,686	613,932			781
Payments to Acquire Equity Method Investments										106,168			50,068
Revenues						$ 654,773			733,741	$ 1,407,329			1,459,508
Capital Gallery [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²	455,000
Contractual Sales Price	$ 253,700
Proceeds from sales of real estate	246,900
Gains on sales of real estate	$ 203,800
Operating Income (Loss)								$ 2,900	3,500			$ 6,300	7,000
New Dominion Technology Park [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²		493,000
Contractual Sales Price		$ 256,000
Proceeds from sales of real estate		254,000
Gains on sales of real estate		$ 192,300
Operating Income (Loss)					$ 1,600				2,100				4,200
number of buildings | Buildings		2
Gateway Center [Member]
Real Estate Properties [Line Items]
Gains on sales of real estate			$ 217,700
Operating Income (Loss)				$ 200					3,100				6,000
425 Fourth Street [Member]
Real Estate Properties [Line Items]
Aggregate purchase price														$ 140,100
Net Rentable Area (in sf) | ft²														804,000
17Fifty Presidents Street [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²																276,000
Leased percentage																100.00%
20 Citypoint [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²															211,000
Leased percentage															62.00%
Capital Gallery [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²	631,000													176,000
Gateway Commons Complex [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²			768,000
Payments to Acquire Equity Method Investments			$ 350,000
Ownership Percentage			50.00%			50.00%	[1]			50.00%	[1]
number of buildings | Buildings			3
Gateway Commons Complex prior to deferred contribution [Member] [Member]
Real Estate Properties [Line Items]
Ownership Percentage			55.00%
Gateway Commons Complex Future Development[Member] [Member]
Real Estate Properties [Line Items]
Ownership Percentage			49.00%
Joint Venture Partner [Member] | Gateway Commons Complex [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²			768,000
Payments to Acquire Equity Method Investments			$ 280,800
Future payment to acquire interest in equity method investment			$ 69,200
Ownership Percentage			50.00%
number of buildings | Buildings			3
Joint Venture Partner [Member] | Gateway Commons Complex Future Development[Member] [Member]
Real Estate Properties [Line Items]
Ownership Percentage			51.00%
Boston Properties Limited Partnership
Real Estate Properties [Line Items]
Proceeds from sales of real estate										$ 506,337			60,398
Gains on sales of real estate						$ 207,241			1,835	626,895			930
Payments to Acquire Equity Method Investments										106,168			50,068
Revenues						$ 654,773			$ 733,741	$ 1,407,329			$ 1,459,508
Boston Properties Limited Partnership | Capital Gallery [Member]
Real Estate Properties [Line Items]
Gains on sales of real estate	$ 207,200
Boston Properties Limited Partnership | New Dominion Technology Park [Member]
Real Estate Properties [Line Items]
Gains on sales of real estate		$ 197,100
Boston Properties Limited Partnership | Gateway Center [Member]
Real Estate Properties [Line Items]
Gains on sales of real estate			$ 222,400
[1] As a result of the partner’s deferred contribution, the Company owns an approximately 55% interest in the joint venture at June 30, 2020. Future development projects will be owned 49% by the Company and 51% by its partner.

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