Investment in Unconsolidated Real Estate Ventures - Investment in Real Estate Ventures and Share of Real Estate Ventures' Income (Loss) (Parenthetical) (Details) $ in Thousands						3 Months Ended		12 Months Ended
		Oct. 18, 2017 USD ($) ft² property	Oct. 17, 2017 USD ($) property	Jan. 31, 2017 USD ($) property	Mar. 02, 2016 USD ($) option term	Dec. 31, 2017 USD ($) ft² property option		Dec. 31, 2017 USD ($) ft² property option		Jan. 05, 2018	Sep. 30, 2017 USD ($) ft²	Sep. 14, 2017 USD ($) ft²	Dec. 31, 2016 USD ($)	Sep. 22, 2016	Jun. 30, 2016	Dec. 31, 2015	Oct. 17, 2014 ft²	Jul. 31, 2014 ft²	Dec. 20, 2011	Jan. 20, 2011
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment Summarized Financial Information Debt						$ 768,912		$ 768,912					$ 997,466
Notional amount						301,548		301,548					328,610
Mortgage notes payable, net						$ 317,216		$ 317,216					$ 321,549
DRA Austin Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage						50.00%		50.00%					50.00%			50.00%
Number of Properties Sold | property		8
Equity Method Investment Summarized Financial Information Debt		$ 151,000
Rentable Square Feet | ft²		1,164,496
Sales Price		$ 333,300
TB-BDN Plymouth Apartments [Member]
Schedule Of Equity Method Investments [Line Items]
Number of Properties Sold | property				398
Sale of ownership interest percentage				50.00%
Sales Price				$ 100,500
Percentage of ownership interests				50.00%
BDN - Al Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage						50.00%	[1],[2],[3]	50.00%	[1],[2],[3]				50.00%			50.00%			50.00%
Debt Instrument, Interest Rate, Effective Percentage	[1],[3]					3.94%		3.94%
Equity Method Investment Summarized Financial Information Debt	[1],[3]					$ 93,117		$ 93,117					$ 131,539
Rentable Square Feet | ft²												874,479
BDN - Al Venture [Member] | 7101 Wisconsin Property Sale [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage												50.00%
Rentable Square Feet | ft²											239,904	230,904
Sales Price											$ 105,700	$ 105,700
HSRE BDN I, LLC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage	[2],[4]					50.00%		50.00%
Equity Method Investment Summarized Financial Information Debt	[4]					$ 110,886		$ 110,886					105,000
Austin Joint Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage		50.00%
Number of Properties Sold | property		8	7			8
Equity Method Investment Summarized Financial Information Debt		$ 150,968	$ 151,000
Rentable Square Feet | ft²		1,164,496				1,570,123		1,570,123									590,881	232,274
Sales Price		$ 333,250
MAP Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage	[2],[5]					50.00%		50.00%
Equity Method Investment Summarized Financial Information Debt	[5]					$ 180,800		$ 180,800					180,800
Seven Tower Bridge [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage	[2],[6]					20.00%		20.00%
Debt Instrument, Interest Rate, Effective Percentage	[6]					3.17%		3.17%
Equity Method Investment Summarized Financial Information Debt	[6]					$ 14,629		$ 14,629					14,710
Seven Tower Bridge [Member] | Subsequent Event [Member]
Schedule Of Equity Method Investments [Line Items]
Sale of ownership interest percentage										20.00%
Four Tower Bridge [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage	[2],[6]					65.00%		65.00%
Debt Instrument, Interest Rate, Effective Percentage	[6]					5.20%		5.20%
Equity Method Investment Summarized Financial Information Debt	[6]					$ 9,749		$ 9,749					9,961
Four Tower Bridge [Member] | Subsequent Event [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage										35.00%
PJP V [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage														25.00%
1000 Chesterbrook Blvd. [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage															50.00%
Brandywine Nineteen Nineteen Ventures [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage						50.00%	[2],[4],[7]	50.00%	[2],[4],[7]											50.00%
Equity Method Investment Summarized Financial Information Debt	[4],[7]					$ 88,860		$ 88,860					$ 79,250
Interest rate protection cap								4.50%
Debt Instrument, Interest Rate Terms								LIBOR + 1.75
4040 Wilson Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage						50.00%	[2],[8]	50.00%	[2],[8]				50.00%
Equity Method Investment Summarized Financial Information Debt	[8]					$ 6,664		$ 6,664					$ 1,004
Mortgage notes payable, net						$ 150,000		$ 150,000
Secured JV Debt [Member] | BDN - Al Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Debt Instrument, Number of Fixed Rate Mortgages						2		2
Secured JV Debt [Member] | BDN - Al Venture [Member] | Fixed rate loan maturing January 1, 2022 [Member]
Schedule Of Equity Method Investments [Line Items]
Debt Instrument, Face Amount						$ 26,600		$ 26,600
Debt Instrument, Interest Rate, Stated Percentage						4.65%		4.65%
Debt Instrument Maturity Date								Jan. 01, 2022
Secured JV Debt [Member] | BDN - Al Venture [Member] | Fixed rate loan maturing August 1, 2019 [Member]
Schedule Of Equity Method Investments [Line Items]
Debt Instrument, Face Amount						$ 66,500		$ 66,500
Debt Instrument, Interest Rate, Stated Percentage						3.22%		3.22%
Debt Instrument Maturity Date								Aug. 01, 2019
Secured JV Debt [Member] | HSRE BDN I, LLC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage					50.00%
Secured JV Debt [Member] | Austin Joint Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Number of Properties Sold | property		8						8
Debt Instrument, Number of Mortgages						3		3
Number of Real Estate Properties Encumbered by Mortgage Debt | property		7
Equity Method Investment Summarized Financial Information Debt		$ 151,000
Secured JV Debt [Member] | Austin Joint Venture [Member] | First mortgage - swapped to fixed [Member]
Schedule Of Equity Method Investments [Line Items]
Debt Instrument, Face Amount						$ 133,600		$ 133,600
Debt Instrument Maturity Date								Nov. 01, 2018
Derivative, Fixed Interest Rate						1.43%		1.43%
Debt Instrument, Interest Rate, Effective Percentage						3.44%		3.44%
Derivative, Basis Spread on Variable Rate						2.01%		2.01%
Secured JV Debt [Member] | Austin Joint Venture [Member] | Second mortgage - swapped to fixed [Member]
Schedule Of Equity Method Investments [Line Items]
Debt Instrument, Face Amount						$ 28,700		$ 28,700
Debt Instrument, Interest Rate, Stated Percentage						4.50%		4.50%
Debt Instrument Maturity Date								Apr. 06, 2019
Secured JV Debt [Member] | Austin Joint Venture [Member] | Third mortgage - swapped to fixed [Member]
Schedule Of Equity Method Investments [Line Items]
Debt Instrument, Face Amount						$ 87,100		$ 87,100
Debt Instrument Maturity Date								Feb. 10, 2020
Derivative, Fixed Interest Rate						1.36%		1.36%
Debt Instrument, Interest Rate, Effective Percentage						3.36%		3.36%
Derivative, Basis Spread on Variable Rate						2.00%		2.00%
Secured JV Debt [Member] | MAP Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Debt Instrument Maturity Date								Feb. 09, 2018
Interest rate protection cap					1.75%
Notional amount					$ 200,800
Debt instrument, number of options to extend maturity date | option					3
Debt instrument, number of successive terms | term					3
Debt instrument, first option extended maturity date								Feb. 09, 2019
Debt instrument, remaining unexercised options extended maturity date								Feb. 09, 2021
Debt instrument, number of options exercised to extend maturity date | option						2		2
Secured JV Debt [Member] | Seven Tower Bridge [Member] | Mortgage Maturing March 2017 [Member]
Schedule Of Equity Method Investments [Line Items]
Debt Instrument, Interest Rate, Stated Percentage						7.00%		7.00%
Debt Instrument Maturity Date								Mar. 01, 2017
Debt Instrument, Number of Fixed Rate Mortgages						2		2
Equity Method Investment Summarized Financial Information Debt						$ 8,000		$ 8,000
Secured JV Debt [Member] | Seven Tower Bridge [Member] | Fixed Rate Loan Maturing September 2025 [Member]
Schedule Of Equity Method Investments [Line Items]
Debt Instrument, Interest Rate, Stated Percentage						3.00%		3.00%
Debt Instrument Maturity Date								Sep. 01, 2025
Equity Method Investment Summarized Financial Information Debt						$ 700		$ 700
Secured JV Debt [Member] | Seven Tower Bridge [Member] | Fixed Rate Loan Maturing February 2017 [Member]
Schedule Of Equity Method Investments [Line Items]
Debt Instrument, Interest Rate, Stated Percentage						4.00%		4.00%
Debt Instrument Maturity Date								Feb. 07, 2018
Equity Method Investment Summarized Financial Information Debt						$ 2,000		$ 2,000
Secured JV Debt [Member] | Seven Tower Bridge [Member] | Fixed Rate Loan Maturing March 2020 [Member]
Schedule Of Equity Method Investments [Line Items]
Debt Instrument, Interest Rate, Stated Percentage						3.25%		3.25%
Debt Instrument Maturity Date								Mar. 11, 2020
Equity Method Investment Summarized Financial Information Debt						$ 3,900		$ 3,900

[1] On October 18, 2017, the DRA (G&I) Austin venture sold eight office properties in Austin, Texas containing 1,164,496 square feet and encumbered by $151.0 million of mortgage debt for a gross sales price of $333.3 million. On September 14, 2017, the BDN-AI Venture completed the sale of 7101 Wisconsin Avenue, an office property containing 239,904 rentable square feet, located in Bethesda, Maryland, for a gross sales price of $105.7 million. On January 31, 2017, the Company sold its 50% interest in TB-BDN Plymouth Apartments, LP. On September 22, 2016, the Company liquidated its 25% ownership interest in PJP V. On June 30, 2016, the Company liquidated its 50% ownership interest in the venture known as 1000 Chesterbrook. The ownership interest in Invesco, L.P. was sold prior to December 31, 2015, and on August 19, 2016, the Company assigned its residual profits interest to the general partner of Invesco. See below for further detail on these dispositions.

[2] Ownership percentage represents the Company’s entitlement to residual distributions after payments of priority returns, where applicable.

[3] See “Brandywine - AI Venture: 7101 Wisconsin Avenue” and “Brandywine - AI Venture: Other Than Temporary Impairment” sections below for information discussing activity that occurred during 2017 relating to this venture.

[4] The basis differences associated with these ventures are allocated between cost and the underlying equity in the net assets of the investee and is accounted for as if the entity were consolidated (i.e., allocated to the Company’s relative share of assets and liabilities with an adjustment to recognize equity in earnings for the appropriate depreciation/amortization). The Company increased its ownership interest in the HSRE-BDN I, LLC venture (also referred to as the “evo at Cira South Venture”) to 50% on March 2, 2016. On June 10, 2016, HSRE-BDN I, LLC refinanced its debt. See “evo at Cira South Venture” section below for further details on these transactions. Also, see “Austin Venture” section below for information discussing this venture’s sale of eight office properties during 2017. Subsequent to December 31, 2017, the Company disposed of its interest in the evo at Cira South Venture. See Note 20, “Subsequent Events,” to the consolidated financial statements for further information.

[5] In order to fulfill interest rate protection requirements, a LIBOR interest rate cap of 1.75% was purchased, effective February 3, 2016 and maturing February 9, 2018, for a notional amount of $200.8 million. There are three options to extend the maturity date of the debt for three successive terms, each year representing a separate option.

[6] Subsequent to December 31, 2017, the Company exchanged its 20% interest in Seven Tower Bridge to acquire the remaining 35% interest in Four Tower Bridge. See Note 20, “Subsequent Events,” to the consolidated financial statements for further information.

[7] The stated rate for the construction loan is LIBOR + 1.75%. To fulfill interest rate protection requirements, an interest rate cap was purchased at 4.50%.

[8] During the fourth quarter of 2017, 4040 Wilson obtained a secured construction loan with a total borrowing capacity of $150.0 million.