Investment in Unconsolidated Joint Ventures (Narrative) (Details) $ in Thousands													3 Months Ended		9 Months Ended
		Sep. 09, 2022 USD ($) ft²	Sep. 08, 2022 USD ($)	Sep. 01, 2022 USD ($) ft²	Aug. 08, 2022 USD ($) ft²	Aug. 07, 2022 USD ($)	Jun. 17, 2022 USD ($)	Jun. 16, 2022 USD ($) ft²	May 13, 2022 USD ($) loanExtension years	Apr. 18, 2022 USD ($) ft²	Mar. 28, 2022 USD ($) loanExtension years	Mar. 27, 2022 USD ($) loanExtension years	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Feb. 02, 2022 ft²	Jan. 18, 2022 ft²	Dec. 31, 2021 USD ($)
Schedule of Equity Method Investments [Line Items]
Mortgage notes payable, net (amounts related to VIEs of $3,271,157 and $3,267,914 at September 30, 2022 and December 31, 2021, respectively)													$ 3,271,157		$ 3,271,157				$ 3,267,914
Payment for Debt Extinguishment or Debt Prepayment Cost															0	$ 185
Loss from early extinguishment of debt													$ 0	$ 0	$ 0	(898)
Gateway Commons Complex [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	[1]												50.00%		50.00%
Gateway Commons Complex [Member] | 651 Gateway | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Ownership Percentage																		50.00%
Platform 16
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	[2]												55.00%		55.00%
Platform 16 | Platform 16 | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Ownership Percentage																	55.00%
Net Rentable Area (in sf) | ft²																	1,100,000
BP/CRF Metropolitan Square LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage													20.00%		20.00%
BP/CRF Metropolitan Square LLC [Member] | Metropolitan Square | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Ownership Percentage											20.00%
Net Rentable Area (in sf) | ft²				657,000
Mortgage notes payable, net (amounts related to VIEs of $3,271,157 and $3,267,914 at September 30, 2022 and December 31, 2021, respectively)											$ 420,000	$ 294,100
Debt Instrument, Description of Variable Rate Basis												the greater of (x) LIBOR or (y) 0.65%
Debt Instrument, Basis Spread on Variable Rate											2.75%	4.75%
Number of extensions | loanExtension											3	2
Extension Option (in years) | years											1	1
Payment for Debt Extinguishment or Debt Prepayment Cost											$ 0
Loss from early extinguishment of debt											(1,300)
Proceeds from mortgage notes payable											100,500
BP/CRF Metropolitan Square LLC [Member] | Metropolitan Square | Unconsolidated Properties | Interest Rate Cap
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Description of Variable Rate Basis				SOFR
Derivative, Notional Amount				$ 420,000
Derivative, Cap Interest Rate				4.50%
Residential Tower Developer LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage													50.00%		50.00%
Residential Tower Developer LLC [Member] | Hub50House [Member] | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Ownership Percentage										50.00%
Proceeds from mortgage notes payable							$ 6,800
Residential Tower Developer LLC [Member] | Hub50House [Member] | Unconsolidated Properties | Interest Rate Swap
Schedule of Equity Method Investments [Line Items]
Derivative, Notional Amount							$ 185,000
Derivative, Forward Interest Rate							4.432%
Residential Tower Developer LLC [Member] | Hub50House [Member] | Unconsolidated Properties | Adjustable Rate Loans
Schedule of Equity Method Investments [Line Items]
Mortgage notes payable, net (amounts related to VIEs of $3,271,157 and $3,267,914 at September 30, 2022 and December 31, 2021, respectively)							$ 185,000	$ 176,700		$ 176,500
Debt Instrument, Basis Spread on Variable Rate							1.35%			2.00%
PR II BXP Reston Gateway LLC
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	[3]												20.00%		20.00%
751 Gateway Holdings LLC
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	[1]												49.00%		49.00%
BNY Tower Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage													50.00%		50.00%
BNY Tower Holdings LLC [Member] | Dock 72 [Member] | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Ownership Percentage					50.00%
Net Rentable Area (in sf) | ft²					669,000
Mortgage notes payable, net (amounts related to VIEs of $3,271,157 and $3,267,914 at September 30, 2022 and December 31, 2021, respectively)					$ 198,400	$ 198,400
Debt Instrument, Description of Variable Rate Basis					the greater of (x) SOFR or (y) 0.25%	LIBOR
Debt Instrument, Basis Spread on Variable Rate					3.10%	3.35%
Debt instrument, borrowing capacity						$ 250,000
Safeco Plaza REIT LLC
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	[4]												33.67%		33.67%
Safeco Plaza REIT LLC | Safeco Plaza | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		33.67%
Net Rentable Area (in sf) | ft²		765,000
Mortgage notes payable, net (amounts related to VIEs of $3,271,157 and $3,267,914 at September 30, 2022 and December 31, 2021, respectively)			$ 250,000
Debt Instrument, Description of Variable Rate Basis		the greater of (x) 2.35% or (y) SOFR	the greater of (x) 2.35% or (y) LIBOR
Debt Instrument, Basis Spread on Variable Rate		2.32%	2.20%
Safeco Plaza REIT LLC | Safeco Plaza | Unconsolidated Properties | Minimum [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		2.35%	2.35%
Safeco Plaza REIT LLC | Safeco Plaza | Unconsolidated Properties | Interest Rate Cap
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Description of Variable Rate Basis		SOFR
Derivative, Notional Amount		$ 250,000
Derivative, Cap Interest Rate		2.50%
Joint Venture Partner [Member] | PR II BXP Reston Gateway LLC | Reston Next Residential | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Payments to Acquire Interest in Joint Venture									$ 500
Joint Venture Partner [Member] | PR II BXP Reston Gateway LLC | Reston Next Residential | Unconsolidated Properties | Scenario, Plan
Schedule of Equity Method Investments [Line Items]
Ownership Percentage									80.00%
Payments to Acquire Interest in Joint Venture									$ 58,700
Joint Venture Partner [Member] | 751 Gateway Holdings LLC | 751 Gateway | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Ownership Percentage								51.00%
Contribution of Property								$ 53,900
Payments to Acquire Interest in Joint Venture								$ 4,900
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Mortgage notes payable, net (amounts related to VIEs of $3,271,157 and $3,267,914 at September 30, 2022 and December 31, 2021, respectively)													$ 3,425,972		$ 3,425,972				$ 3,214,961
Loss from early extinguishment of debt													$ 0	$ 0	$ (1,327)	$ 0
Unconsolidated Joint Ventures [Member] | PR II BXP Reston Gateway LLC | Reston Next Residential | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Contribution of Property									11,300
Unconsolidated Joint Ventures [Member] | PR II BXP Reston Gateway LLC | Reston Next Residential | Unconsolidated Properties | Scenario, Plan
Schedule of Equity Method Investments [Line Items]
Payments to Acquire Interest in Joint Venture									$ 3,500
Unconsolidated Joint Ventures [Member] | PR II BXP Reston Gateway LLC | Reston Next Residential | Unconsolidated Properties | Adjustable Rate Loans
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Description of Variable Rate Basis									SOFR
Debt Instrument, Basis Spread on Variable Rate									2.00%
Number of extensions | loanExtension									2
Extension Option (in years) | years									1
Debt Instrument, Unused Borrowing Capacity, Amount									$ 140,000
Unconsolidated Joint Ventures [Member] | 751 Gateway Holdings LLC | 751 Gateway | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Ownership Percentage								49.00%
Contribution of Property								$ 53,900
Payments to Acquire Interest in Joint Venture								$ 2,600
Company's Share [Member] | BP/CRF Metropolitan Square LLC [Member] | Metropolitan Square
Schedule of Equity Method Investments [Line Items]
Proceeds from mortgage notes payable											$ 20,100
Company's Share [Member] | Residential Tower Developer LLC [Member] | Hub50House [Member] | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Proceeds from mortgage notes payable							$ 3,400
Company's Share [Member] | PR II BXP Reston Gateway LLC | Reston Next Residential | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Ownership Percentage									96.00%
Company's Share [Member] | PR II BXP Reston Gateway LLC | Reston Next Residential | Unconsolidated Properties | Scenario, Plan
Schedule of Equity Method Investments [Line Items]
Ownership Percentage									20.00%
Construction | Gateway Commons Complex [Member] | 651 Gateway | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Net Rentable Area (in sf) | ft²																		327,000
Construction | Platform 16 | Platform 16 | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Net Rentable Area (in sf) | ft²																	390,000
Construction | 751 Gateway Holdings LLC | 751 Gateway | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Net Rentable Area (in sf) | ft²								231,000
Construction | Unconsolidated Joint Ventures [Member] | PR II BXP Reston Gateway LLC | Reston Next Residential | Unconsolidated Properties | Scenario, Plan
Schedule of Equity Method Investments [Line Items]
Number of Units in Real Estate Property									508
Residential Building | Residential Tower Developer LLC [Member] | Hub50House [Member] | Unconsolidated Properties
Schedule of Equity Method Investments [Line Items]
Net Rentable Area (in sf) | ft²										320,000
Number of Units in Real Estate Property										440

[1] On June 16, 2022, in accordance with the Gateway Commons joint venture agreement, 751 Gateway was segregated into a new single-purpose joint venture.

[2] This entity is a VIE (See Note 2).

[3] The Company’s partner will fund required capital until its aggregate investment is approximately 80% of all capital contributions; thereafter, the partners will fund required capital according to their percentage interests.

[4] The Company’s ownership includes (1) a 33.0% direct interest in the joint venture, and (2) an additional 1% interest in each of the two entities through which each partner owns its interest in the joint venture.