Investment in Unconsolidated Real Estate Ventures (Textual) (Details)				3 Months Ended		9 Months Ended
		Sep. 14, 2017 USD ($) ft² property	Jan. 31, 2017 USD ($) property	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) property	Sep. 30, 2016 USD ($)	Sep. 30, 2017	Sep. 30, 2017 property	Sep. 30, 2017 ft²	Sep. 30, 2017 Real_Estate_Investment	Sep. 30, 2017 a	Sep. 30, 2017 Apartment_unit	Dec. 31, 2016 USD ($)	Jul. 10, 2012 ft² property	Dec. 20, 2011 ft² property
Schedule Of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method				$ 236,313,000		$ 236,313,000								$ 281,331,000
Management fees				6,918,000	$ 6,248,000	20,483,000	$ 17,691,000
Accounts receivable				14,785,000		14,785,000								12,446,000
Proceeds from the sale of properties						114,821,000	758,931,000
Cash proceeds after the payment of share of the debt and closing costs						59,022,000	20,551,000
Net gain on Real Estate Venture transactions				13,758,000	10,472,000	28,340,000	19,529,000
1919 Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Guarantees, maximum exposure amount				88,900,000		$ 88,900,000
TB-BDN Plymouth Apartments [Member]
Schedule Of Equity Method Investments [Line Items]
Gross sales price			$ 100,500,000
Sale of ownership interest percentage			50.00%
Allocated gross sales value of ownership interest, percentage			50.00%
Number of Properties Sold | property			398
Guarantee obligations cancelled			$ 3,200,000
Cash proceeds after the payment of share of the debt and closing costs			27,200,000
Net gain on Real Estate Venture transactions			14,600,000
TB-BDN Plymouth Apartments [Member] | Pennsylvania [Member]
Schedule Of Equity Method Investments [Line Items]
Construction Loan			54,000,000
Office Properties [Member]
Schedule Of Equity Method Investments [Line Items]
Number of Properties Sold | property						12
Office Properties [Member] | 7101 Wisconsin Property Sale [Member]
Schedule Of Equity Method Investments [Line Items]
Rentable Square Feet | ft²															510,202	587,317
Number of office properties acquired | property															3
Real Estate Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Management fees				1,700,000	1,700,000	$ 4,900,000	4,800,000
Accounts receivable				1,100,000		1,100,000								1,400,000
Real Estate Venture [Member] | Lease Commission Income [Member]
Schedule Of Equity Method Investments [Line Items]
Management fees				400,000	$ 800,000	3,300,000	$ 1,900,000
Unconsolidated Real Estate Ventures [Member]
Schedule Of Equity Method Investments [Line Items]
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment											13
Investment in Real Estate Ventures, equity method	[1]			236,313,000		236,313,000								$ 281,331,000
Real estate ventures aggregate indebtedness to third parties				915,000,000		915,000,000
Unconsolidated Real Estate Ventures [Member] | TB-BDN Plymouth Apartments [Member]
Schedule Of Equity Method Investments [Line Items]
Equity in income (loss) of Real Estate Ventures			$ 12,600,000
Unconsolidated Real Estate Ventures [Member] | Minimum [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage								20.00%
Unconsolidated Real Estate Ventures [Member] | Maximum [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage								70.00%
Unconsolidated Real Estate Ventures [Member] | Seven Real Estate Ventures [Member] | Office Properties [Member]
Schedule Of Equity Method Investments [Line Items]
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment											7
Rentable Square Feet | ft²										7,800,000
Unconsolidated Real Estate Ventures [Member] | Four Real Estate Ventures [Member] | Land Held For Development [Member]
Schedule Of Equity Method Investments [Line Items]
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment											4
Acreage of land | a												5.7
Unconsolidated Real Estate Ventures [Member] | Two Other Real Estate Ventures [Member] | Residential Tower One [Member]
Schedule Of Equity Method Investments [Line Items]
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment											1
Number of Property units									345				345
Unconsolidated Real Estate Ventures [Member] | Two Other Real Estate Ventures [Member] | Residential Tower Two [Member]
Schedule Of Equity Method Investments [Line Items]
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment											1
Number of Property units									321				321
BDN - Al Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method				44,300,000		44,300,000
Rentable Square Feet | ft²		874,479
Equity method investment percentage																50.00%
Number of office properties owned | property		5							5
Cost of equity method investment		$ 49,100,000		49,100,000		49,100,000
Equity method investment other than temporary impairment charge				4,800,000		$ 4,800,000
Impairment Hold Period						10 years
Residual Capitalization Rates						7.50%
BDN - Al Venture [Member] | 7101 Wisconsin Property Sale [Member]
Schedule Of Equity Method Investments [Line Items]
Rentable Square Feet | ft²		230,904
Equity method investment percentage		50.00%
Gross sales price		$ 105,700,000
Property encumbered at the time of venture sale		37,400,000
Debt instrument prepayment penalty		800,000
Cash proceeds received from sale of venture		63,600,000
Proceeds from the sale of properties		31,800,000
Gains on sales of real estate		$ 13,800,000
BDN - Al Venture [Member] | Current Creek Investments, LLC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage																50.00%
BDN - Al Venture [Member] | Minimum [Member]
Schedule Of Equity Method Investments [Line Items]
Discount Rates						7.80%
BDN - Al Venture [Member] | Maximum [Member]
Schedule Of Equity Method Investments [Line Items]
Discount Rates						8.50%
BDN - Al Venture [Member] | Office Properties [Member] | 7101 Wisconsin Property Sale [Member]
Schedule Of Equity Method Investments [Line Items]
Number of office properties owned | property																6
Number of office properties upon formation | property																3
Number of office properties subsequent to formation | property																3
evo at Cira Centre South Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Guarantees, maximum exposure amount				55,400,000		$ 55,400,000
PJP VII [Member]
Schedule Of Equity Method Investments [Line Items]
Guarantees, maximum exposure amount				$ 400,000		$ 400,000

[1] This amount includes the effect of the basis difference between the Company's historical cost basis and the basis recorded at the Real Estate Venture level, which is typically amortized over the life of the related assets and liabilities. Basis differentials occur from the impairment of investments, purchases of third party interests in existing Real Estate Ventures and upon the transfer of assets that were previously owned by the Company into a Real Estate Venture. In addition, certain acquisition, transaction and other costs may not be reflected in the net assets at the Real Estate Venture level.