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Real Estate (Narrative) (Details) $ in Thousands								1 Months Ended	3 Months Ended									4 Months Ended		5 Months Ended	6 Months Ended	12 Months Ended
	Dec. 20, 2019 USD ($)	Sep. 20, 2019 USD ($) ft² a	Jul. 16, 2019 USD ($) ft²	Jun. 28, 2019 USD ($) ft²	Jun. 03, 2019 USD ($) ft²	May 09, 2019 USD ($) ft²	Jan. 24, 2019 USD ($) ft² a	Jan. 23, 2019 USD ($)	Dec. 31, 2019 USD ($) ft²		Sep. 30, 2019 USD ($) ft²	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) ft²		Jun. 02, 2019 USD ($)	Jun. 27, 2019 USD ($)	Dec. 31, 2019 USD ($) ft²		Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Oct. 24, 2019 ft²	Aug. 27, 2019 USD ($) ft² Real_Estate_Properties	Jun. 01, 2019 ft²	Apr. 18, 2019 USD ($)	Jan. 10, 2019 USD ($) ft²
Real Estate Properties [Line Items]
Accelerated depreciation																						$ 677,764		$ 645,649	$ 617,547
Right-of-use assets - finance leases									$ 237,394									$ 237,394				237,394
Lease liabilities - finance leases									224,042									$ 224,042				224,042
Proceeds from sales of real estate																						90,824		455,409	29,810
Gains (loss) on sales of real estate																						709		182,356	7,663
Payments to Acquire Equity Method Investments																						87,392		345,717	109,015
Revenues									$ 757,501		$ 743,553	$ 733,741	$ 725,767	$ 705,157	$ 686,284	$ 664,484	$ 661,151					$ 2,960,562		2,717,076	2,602,076
2600 Tower Oaks Boulevard [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²							179,000
Contractual Sales Price							$ 22,700
Proceeds from sales of real estate							21,400
Gains (loss) on sales of real estate							$ (600)
Impairment losses																								3,100
Operating Income (Loss)								$ (200)																(600)	(100)
One Tower Center [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²					410,000
Contractual Sales Price																													$ 38,000
Proceeds from sales of real estate					$ 36,600
Gains (loss) on sales of real estate					$ (800)
Impairment losses													24,000
Operating Income (Loss)																				$ (900)				(2,700)	(3,500)
164 Lexington Road [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²				64,000
Contractual Sales Price				$ 4,000
Proceeds from sales of real estate				3,800
Gains (loss) on sales of real estate				2,500
Operating Income (Loss)																					$ (100)			(200)	(200)
Washingtonian North [Member]
Real Estate Properties [Line Items]
Contractual Sales Price	$ 7,800
Proceeds from sales of real estate	7,300
Gains (loss) on sales of real estate	$ (100)
Impairment losses																								1,800
325 Main Street
Real Estate Properties [Line Items]
Term of Lease Signed (in years)						15-year
Net Rentable Area (in sf) | ft²						115,000			115,000									115,000				115,000
Accelerated depreciation						$ 9,900
20 CityPoint
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²																												211,000
200 West Street
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²											261,000
145 Broadway
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²																										483,000
Platform 16
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²							1,100,000
Lessee, Finance Lease, Term of Contract							65 years
Area of Land | a							5.6
Finance lease fixed purchase price							$ 134,800
Right-of-use assets - finance leases							122,600
Lease liabilities - finance leases							$ 122,563
2100 Pennsylvania Avenue
Real Estate Properties [Line Items]
Term of Lease Signed (in years)			16-year
Square Footage Of Signed Lease | ft²			300,000
Net Rentable Area (in sf) | ft²			470,000
Lessee, Finance Lease, Term of Contract			75 years
Right-of-use assets - finance leases			$ 185,100
Prepaid Rent			15,000
Lease liabilities - finance leases			164,977
Finance lease - right of use asset -prepaid rent			15,000
Finance lease - right of use asset - initial direct costs			$ 5,100
Lessee, finance lease, term of amortization			75 years
Carnegie Center
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²																														1,700,000
Aggregate purchase price																														$ 51,500
Deferred purchase price																														$ 8,600
880 & 890 Winter Street [Member]
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²																											392,000
Aggregate purchase price																											$ 106,000
Number of real estate properties | Real_Estate_Properties																											2
Operating Income (Loss)																		$ (600)
Revenues																		4,900
Boston Properties Limited Partnership
Real Estate Properties [Line Items]
Accelerated depreciation																						$ 669,956		637,891
Right-of-use assets - finance leases									$ 237,394									237,394				237,394
Lease liabilities - finance leases									224,042									$ 224,042				224,042
Proceeds from sales of real estate																						90,824		455,409	29,810
Gains (loss) on sales of real estate																						858		190,716	8,240
Impairment losses																									0
Payments to Acquire Equity Method Investments																						87,392		345,717	109,015
Revenues									$ 757,501		$ 743,553	$ 733,741	725,767	$ 705,157	$ 686,284	$ 664,484	$ 661,151					$ 2,960,562		2,717,076	$ 2,602,076
Boston Properties Limited Partnership | 2600 Tower Oaks Boulevard [Member]
Real Estate Properties [Line Items]
Impairment losses																								$ 1,500
Boston Properties Limited Partnership | One Tower Center [Member]
Real Estate Properties [Line Items]
Impairment losses													$ 22,300
Boston Properties Limited Partnership | 164 Lexington Road [Member]
Real Estate Properties [Line Items]
Gains (loss) on sales of real estate				$ 2,600
Boston Properties Limited Partnership | 325 Main Street
Real Estate Properties [Line Items]
Accelerated depreciation						$ 9,500
Future Office Building [Member] | 325 Main Street
Real Estate Properties [Line Items]
Square Footage Of Signed Lease | ft²						379,000
Net Rentable Area (in sf) | ft²						420,000			420,000									420,000				420,000
Future Retail Building [Member] | 325 Main Street
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²						41,000			41,000									41,000				41,000
Office Properties Under Construction [Member] | 200 West Street
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²											126,000
Platform 16
Real Estate Properties [Line Items]
Net Rentable Area (in sf) | ft²		1,100,000
Lessee, Finance Lease, Term of Contract		65 years
Area of Land | a		5.6
Finance lease fixed purchase price		$ 134,800
Payments to Acquire Equity Method Investments		$ 28,200
Ownership Percentage		55.00%							55.00%	[1]								55.00%	[1]			55.00%	[1]
Joint Venture Partner [Member] | Platform 16
Real Estate Properties [Line Items]
Payments to Acquire Equity Method Investments		$ 23,100
Ownership Percentage		45.00%
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