Investment in Unconsolidated Real Estate Ventures (Textual) (Details) ft² in Millions			3 Months Ended		6 Months Ended
		Jan. 31, 2017 USD ($) property	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) property	Jun. 30, 2016 USD ($)	Jun. 30, 2017	Jun. 30, 2017 property	Jun. 30, 2017 ft²	Jun. 30, 2017 Real_Estate_Investment	Jun. 30, 2017 a	Jun. 30, 2017 Apartment_unit	Dec. 31, 2016 USD ($)
Schedule Of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method			$ 262,107,000		$ 262,107,000								$ 281,331,000
Management fees			7,080,000	$ 6,208,000	13,565,000	$ 11,443,000
Accounts receivable			13,151,000		13,151,000								12,446,000
Cash proceeds after the payment of share of the debt and closing costs					27,230,000	4,812,000
Net gain on Real Estate Venture transactions			0	3,128,000	14,582,000	9,057,000
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]
Sale of ownership interest percentage		50.00%
Gross sales price		$ 100,500,000
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
Real Estate Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Management fees			1,600,000	1,600,000	$ 3,200,000	3,100,000
Accounts receivable			2,000,000	1,400,000	2,000,000	1,400,000
Real Estate Venture [Member] | Lease Commission Income [Member]
Schedule Of Equity Method Investments [Line Items]
Management fees			1,500,000	$ 500,000	2,800,000	$ 1,300,000
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]		262,107,000		262,107,000								$ 281,331,000
Real estate ventures aggregate indebtedness to third parties			952,700,000		952,700,000
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									8.1
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
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.