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Real Estate (Narrative) (Details) $ in Thousands						1 Months Ended	2 Months Ended	6 Months Ended	12 Months Ended
	Dec. 16, 2020 USD ($)	Nov. 03, 2020 ft²	Jun. 25, 2020 USD ($) ft²	Feb. 20, 2020 USD ($) ft² Buildings	Jan. 28, 2020 USD ($) ft² Buildings	Jan. 27, 2020 USD ($)	Feb. 19, 2020 USD ($)	Jun. 24, 2020 USD ($)	Dec. 31, 2020 USD ($)		Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Aug. 15, 2020 ft² apartments	Jul. 31, 2020 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									$ 519,303		$ 90,824	$ 455,409
Gains on sales of real estate									618,982		709	182,356
Payments to Acquire Equity Method Investments									172,436		87,392	345,717
Revenues									$ 2,765,686		2,960,562	2,717,076
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			246,600
Gains on sales of real estate			$ 203,500
Operating Income (Loss)								$ 6,300			15,000	14,500
Crane Meadow
Real Estate Properties [Line Items]
Contractual Sales Price	$ 14,300
Proceeds from sales of real estate	14,200
Gains on sales of real estate	$ 5,200
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				6,800	8,200
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					10,400	8,600
425 Fourth Street [Member]
Real Estate Properties [Line Items]
Aggregate purchase price															$ 140,100
Net Rentable Area (in sf) | ft²															850,000
17Fifty Presidents Street
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²																	276,000
Leased percentage																	100.00%
20 Citypoint
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²																211,000
Leased percentage									100.00%
Capital Gallery [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²			631,000												176,000
759 Harrison Street [Member]
Real Estate Properties [Line Items]
Aggregate purchase price														$ 4,500
200 West Street
Real Estate Properties [Line Items]
Leased percentage		100.00%
Lessor, Operating Lease, Lease Not yet Commenced, Description		10-year
Lessor Operating Lease Lease Not Yet Commenced Leased SF | ft²		138,000
Residential Building | The Skylyne
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²													331,000
Number of Units in Real Estate Property | apartments													402
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]
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]
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									$ 519,303		90,824	455,409
Gains on sales of real estate									631,945		858	190,716
Payments to Acquire Equity Method Investments									172,436		87,392	345,717
Revenues									$ 2,765,686		$ 2,960,562	$ 2,717,076
Boston Properties Limited Partnership | Capital Gallery [Member]
Real Estate Properties [Line Items]
Gains on sales of real estate			$ 207,000
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 December 31, 2020. Future development projects will be owned 49% by the Company and 51% by its partner.

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