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Investment in Unconsolidated Real Estate Ventures (Textual) (Details) |  |  | 3 Months Ended | 9 Months Ended |  |  |  |
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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.

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