Investment in Unconsolidated Real Estate Ventures - Summary of Results of Operations of Real Estate Ventures with Interests (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2017	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015	Dec. 20, 2011
Schedule Of Equity Method Investments [Line Items]
Equity in income (loss) of Real Estate Ventures			$ (8,306)		$ (11,503)		$ (811)
DRA Austin Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue			85,500		85,749		78,367
Operating expenses			(35,997)		(37,643)		(33,194)
Provision for impairment	[1]				0
Interest expense, net			(13,985)		(15,052)		(14,025)
Depreciation and amortization			(34,026)		(38,365)		(36,809)
Loss on extinguishment of debt			(2,613)
Net income (loss)			$ (1,121)		$ (5,311)	[2]	$ (5,661)
Ownership interest %			50.00%		50.00%		50.00%
Company's share of net income (loss)			$ (560)		$ (2,656)		$ (2,831)
Other-than-temporary impairment	[3]		0
Basis adjustments and other			(429)		776		1,596
Equity in income (loss) of Real Estate Ventures			(989)		(1,880)		(1,235)
Brandywine - AI Venture LLC [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue			29,500		31,047		29,428
Operating expenses			(12,298)		(13,654)		(11,872)
Provision for impairment	[1]				(10,476)
Interest expense, net			(4,707)		(5,825)		(5,310)
Depreciation and amortization			(11,428)		(12,811)		(12,678)
Loss on extinguishment of debt			(811)
Net income (loss)			$ 256		$ (11,719)	[2]	$ (432)
Ownership interest %			50.00%	[4],[5],[6]	50.00%		50.00%	50.00%
Company's share of net income (loss)			$ 128		$ (5,860)		$ (216)
Other-than-temporary impairment		$ (4,800)	(4,844)	[3]
Basis adjustments and other			251		(35)		(13)
Equity in income (loss) of Real Estate Ventures			(4,465)	[4],[6]	(5,895)	[4],[6]	(229)
Other [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue			101,271		97,656		57,133
Operating expenses			(51,045)		(51,895)		(25,070)
Provision for impairment	[1]				0
Interest expense, net			(21,521)		(19,437)		(15,249)
Depreciation and amortization			(32,986)		(34,562)		(18,613)
Loss on extinguishment of debt			0
Net income (loss)			$ (4,281)		$ (8,238)	[2]	$ (1,799)
Ownership interest %	[7]		0.00%		0.00%		0.00%
Company's share of net income (loss)			$ (1,432)		$ (3,985)		$ 455
Other-than-temporary impairment	[3]		0
Basis adjustments and other			(1,420)		257		198
Equity in income (loss) of Real Estate Ventures			(2,852)		(3,728)		653
Unconsolidated Real Estate Ventures [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue			216,271		214,452		164,928
Operating expenses			(99,340)		(103,192)		(70,136)
Provision for impairment	[1]				(10,476)
Interest expense, net			(40,213)		(40,314)		(34,584)
Depreciation and amortization			(78,440)		(85,738)		(68,100)
Loss on extinguishment of debt			(3,424)
Net income (loss)			$ (5,146)		$ (25,268)	[2]	$ (7,892)
Ownership interest %			0.00%		0.00%		0.00%
Company's share of net income (loss)			$ (1,864)		$ (12,501)		$ (2,592)
Other-than-temporary impairment	[3]		(4,844)
Basis adjustments and other			(1,598)		998		1,781
Equity in income (loss) of Real Estate Ventures			$ (8,306)		$ (11,503)		$ (811)

[1] See “Brandywine - AI Venture: Station Square Impairment” section below.

[2] During the year ended December 31, 2016, there were $7.1 million of acquisition deal costs related to the formation of the MAP Venture.

[3] See “Brandywine - AI Venture: Other Than Temporary Impairment” section below.

[4] 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.

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

[6] 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.

[7] The Company’s unconsolidated ownership interests range from 20% to 70%, subject to specified priority allocations of distributable cash in certain of the Real Estate Ventures.