Investment in Unconsolidated Real Estate Ventures - Investment in Real Estate Ventures and Share of Real Estate Ventures' Income (Loss) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015	Sep. 22, 2016	Jun. 30, 2016	Jan. 29, 2016	May 29, 2015	Dec. 20, 2011	Jan. 20, 2011
Schedule Of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method		$ 194,621		$ 281,331		$ 241,004
Company’s share of income (loss) (Company’s basis)		(8,306)		(11,503)		$ (811)
Equity Method Investments including net liabilities				281,331
Equity Method Investment Summarized Financial Information Debt		$ 768,912		$ 997,466
Brandywine - AI Venture LLC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		50.00%	[1],[2],[3]	50.00%		50.00%					50.00%
Investment in Real Estate Ventures, equity method	[1],[3]	$ 43,560		$ 67,809
Company’s share of income (loss) (Company’s basis)		(4,465)	[1],[3]	(5,895)	[1],[3]	$ (229)
Equity Method Investment Summarized Financial Information Debt	[1],[3]	$ 93,117		131,539
Debt Instrument, Interest Rate, Effective Percentage	[1],[3]	3.94%
DRA (G&I) Austin [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1],[2],[4]	50.00%
Investment in Real Estate Ventures, equity method	[1],[4]	$ 13,972		52,886
Company’s share of income (loss) (Company’s basis)	[1],[4]	(989)		(1,880)
Equity Method Investment Summarized Financial Information Debt	[1],[4]	$ 249,481		405,734
Debt Instrument, Interest Rate, Effective Percentage	[1],[4]	3.51%
MAP Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[2],[5]	50.00%
Investment in Real Estate Ventures, equity method	[5]	$ 15,450		20,893
Company’s share of income (loss) (Company’s basis)	[5]	(3,443)		(4,218)
Equity Method Investment Summarized Financial Information Debt	[5]	$ 180,800		180,800
Debt Instrument, Interest Rate Terms	[5]	L+6.25%
Debt Instrument, Maturity Date, Month and Year	[5]	2018-02
Four Tower Bridge [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[2],[6]	65.00%
Investment in Real Estate Ventures, equity method	[6]	$ 3,032		2,286
Company’s share of income (loss) (Company’s basis)	[6]	746		602
Equity Method Investment Summarized Financial Information Debt	[6]	$ 9,749		9,961
Debt Instrument, Interest Rate, Effective Percentage	[6]	5.20%
Debt Instrument, Maturity Date, Month and Year	[6]	2021-02
PJP VII [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[2]	25.00%
Investment in Real Estate Ventures, equity method		$ 1,156		980
Company’s share of income (loss) (Company’s basis)		175		233
Equity Method Investment Summarized Financial Information Debt		$ 4,792		4,956
Debt Instrument, Interest Rate Terms		L+2.65%
Debt Instrument, Maturity Date, Month and Year		2019-12
PJP Two [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[2]	30.00%
Investment in Real Estate Ventures, equity method		$ 604		532
Company’s share of income (loss) (Company’s basis)		72		97
Equity Method Investment Summarized Financial Information Debt		$ 2,564		2,893
Debt Instrument, Interest Rate, Effective Percentage		6.12%
Debt Instrument, Maturity Date, Month and Year		2023-11
PJP VI [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[2]	25.00%
Investment in Real Estate Ventures, equity method		$ 179		142
Company’s share of income (loss) (Company’s basis)		38		97
Equity Method Investment Summarized Financial Information Debt		$ 7,370		7,652
Debt Instrument, Interest Rate, Effective Percentage		6.08%
Debt Instrument, Maturity Date, Month and Year		2023-04
1000 Chesterbrook Blvd. [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage								50.00%
Company’s share of income (loss) (Company’s basis)	[1]			160
PJP V [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage							25.00%
Company’s share of income (loss) (Company’s basis)	[1]			127
Invesco, L.P. [Member]
Schedule Of Equity Method Investments [Line Items]
Company’s share of income (loss) (Company’s basis)	[1]			261
Coppell Associates [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage									50.00%
Company’s share of income (loss) (Company’s basis)				12
HSRE BDN I, LLC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[2],[4]	50.00%
Investment in Real Estate Ventures, equity method	[4]	$ 17,671		21,228
Company’s share of income (loss) (Company’s basis)	[4]	449		843
Equity Method Investment Summarized Financial Information Debt	[4]	$ 110,886		105,000
Debt Instrument, Interest Rate Terms	[4]	L+2.25%
Debt Instrument, Maturity Date, Month and Year	[4]	2019-10
Brandywine Nineteen Nineteen Ventures [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		50.00%	[2],[4],[7]									50.00%
Investment in Real Estate Ventures, equity method	[4],[7]	$ 22,268		27,462
Company’s share of income (loss) (Company’s basis)	[4],[7]	(194)		(1,529)
Equity Method Investment Summarized Financial Information Debt	[4],[7]	$ 88,860		79,250
Debt Instrument, Interest Rate Terms	[4],[7]	L+1.75%
Debt Instrument, Maturity Date, Month and Year	[4],[7]	2018-10
TB-BDN Plymouth Apartments [Member]
Schedule Of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method	[1]			12,450
Company’s share of income (loss) (Company’s basis)	[1]	$ 99		119
Equity Method Investment Summarized Financial Information Debt	[1]			$ 53,967
4040 Wilson Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		50.00%	[2],[8]	50.00%
Investment in Real Estate Ventures, equity method	[8]	$ 37,179		$ 36,356
Company’s share of income (loss) (Company’s basis)	[8]	(255)		(270)
Equity Method Investment Summarized Financial Information Debt	[8]	$ 6,664		1,004
Debt Instrument, Interest Rate Terms	[8]	L+2.75%
Debt Instrument, Maturity Date, Month and Year	[8]	2021-12
51 N 50 Patterson [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		70.00%	[2]							70.00%
Investment in Real Estate Ventures, equity method		$ 21,212		20,318
Company’s share of income (loss) (Company’s basis)		(176)		(114)
Equity Method Investment Summarized Financial Information Debt		$ 0		0
Seven Tower Bridge [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[2],[6]	20.00%
Investment in Real Estate Ventures, equity method	[6]	$ 471		685
Company’s share of income (loss) (Company’s basis)	[6]	(214)		(133)
Equity Method Investment Summarized Financial Information Debt	[6]	$ 14,629		14,710
Debt Instrument, Interest Rate, Effective Percentage	[6]	3.17%
1250 First Street Office [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		70.00%	[2]							70.00%
Investment in Real Estate Ventures, equity method		$ 17,867		17,304
Company’s share of income (loss) (Company’s basis)		(149)		(15)
Equity Method Investment Summarized Financial Information Debt		$ 0		$ 0

[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.