Investment in Unconsolidated Ventures - Investment in Real Estate Ventures and Share of Real Estate Ventures' Income (Loss) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013	Dec. 30, 2015	May. 29, 2015	Jan. 20, 2011
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1]	0.00%
Investment in Real Estate Ventures, at equity		$ 241,004		$ 225,004		$ 180,512
Equity Method Investments including net liabilities		239,874		223,790
Company’s share of income (loss) (Company’s basis)		(811)		(790)		$ 3,664
Equity Method Investment Summarized Financial Information Debt		$ 794,571		965,077
Debt Instrument, Interest Rate, Effective Percentage		0.00%
Brandywine - Al Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1]	50.00%
Investment in Real Estate Ventures, at equity		$ 50,760		45,712
Company’s share of income (loss) (Company’s basis)		(229)		(185)
Equity Method Investment Summarized Financial Information Debt		$ 132,717		133,843
Debt Instrument, Interest Rate, Effective Percentage		3.94%
Austin Joint Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1],[2]	50.00%
Investment in Real Estate Ventures, at equity	[2]	$ 60,427		40,374
Company’s share of income (loss) (Company’s basis)	[2]	(1,235)		(574)
Equity Method Investment Summarized Financial Information Debt	[2]	$ 410,066		382,100
Debt Instrument, Interest Rate, Effective Percentage	[2]	3.38%
One Thousand Chesterbrook Blvd. [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1]	50.00%
Investment in Real Estate Ventures, at equity		$ 1,895		2,152
Company’s share of income (loss) (Company’s basis)		117		388
Equity Method Investment Summarized Financial Information Debt		$ 23,610		24,251
Debt Instrument, Interest Rate, Effective Percentage		4.75%
Four Tower Bridge [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1]	65.00%
Investment in Real Estate Ventures, at equity		$ 1,684		1,474
Company’s share of income (loss) (Company’s basis)		211		(144)
Equity Method Investment Summarized Financial Information Debt		$ 10,162		10,353
Debt Instrument, Interest Rate, Effective Percentage		5.20%
PJP VII [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1]	25.00%
Investment in Real Estate Ventures, at equity		$ 872		911
Company’s share of income (loss) (Company’s basis)		211		177
Equity Method Investment Summarized Financial Information Debt		$ 5,621		5,785
Debt Instrument, Interest Rate Terms		0
PJP Two [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1]	30.00%
Investment in Real Estate Ventures, at equity		$ 435		403
Company’s share of income (loss) (Company’s basis)		32		73
Equity Method Investment Summarized Financial Information Debt		$ 3,201		3,492
Debt Instrument, Interest Rate, Effective Percentage		6.12%
PJP Five [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1]	25.00%
Investment in Real Estate Ventures, at equity		$ 305		216
Company’s share of income (loss) (Company’s basis)		189		166
Equity Method Investment Summarized Financial Information Debt		$ 5,035		5,242
Debt Instrument, Interest Rate, Effective Percentage		6.47%
PJP Six [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1]	25.00%
Investment in Real Estate Ventures, at equity		$ 45		45
Company’s share of income (loss) (Company’s basis)		151		(67)
Equity Method Investment Summarized Financial Information Debt		$ 7,918		8,168
Debt Instrument, Interest Rate, Effective Percentage		6.08%
Invesco, L.P. [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1],[3]	0.00%
Investment in Real Estate Ventures, at equity	[3]	$ 0		0
Company’s share of income (loss) (Company’s basis)	[3]	349		989
Equity Method Investment Summarized Financial Information Debt	[3]	$ 0		0
Debt Instrument, Interest Rate, Effective Percentage	[3]	0.00%
Debt Instrument, Interest Rate Terms	[3]	0
Broadmoor Austin Associates [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1],[3]	0.00%
Investment in Real Estate Ventures, at equity	[3]	$ 0		65,407
Company’s share of income (loss) (Company’s basis)	[3]	(377)		(460)
Equity Method Investment Summarized Financial Information Debt	[3]	$ 0		53,516
Debt Instrument, Interest Rate, Effective Percentage	[3]	0.00%
Debt Instrument, Interest Rate Terms	[3]	0
GI Interchange Office LLC (DRA — N. PA) [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1],[3]	0.00%
Investment in Real Estate Ventures, at equity	[3]	$ 0		0
Company’s share of income (loss) (Company’s basis)	[3]	0		(658)
Equity Method Investment Summarized Financial Information Debt	[3]	$ 0		174,595
Debt Instrument, Interest Rate, Effective Percentage	[3]	0.00%
Coppell Associates [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1],[4]	50.00%
Investment in Real Estate Ventures, at equity	[4]	$ (1,130)		(1,214)
Company’s share of income (loss) (Company’s basis)	[4]	84		(45)
Equity Method Investment Summarized Financial Information Debt	[4]	$ 15,515		15,747
Debt Instrument, Interest Rate, Effective Percentage	[4]	5.75%
HSRE, LLC [Member] [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1],[2]	30.00%
Investment in Real Estate Ventures, at equity	[2]	$ 15,003		14,314
Company’s share of income (loss) (Company’s basis)	[2]	(188)		67
Equity Method Investment Summarized Financial Information Debt	[2]	$ 95,562		90,245
Debt Instrument, Interest Rate Terms	[2]	0
4040 Wilson Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1]	50.00%
Investment in Real Estate Ventures, at equity		$ 36,626		27,415
Company’s share of income (loss) (Company’s basis)		(106)		(132)
Equity Method Investment Summarized Financial Information Debt		$ 0		0
Debt Instrument, Interest Rate, Effective Percentage		0.00%
TB-BDN Plymouth Apartments [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1]	50.00%
Investment in Real Estate Ventures, at equity		$ 12,338		12,720
Company’s share of income (loss) (Company’s basis)		(252)		(83)
Equity Method Investment Summarized Financial Information Debt		$ 50,964		29,481
Debt Instrument, Interest Rate Terms		0
Residence Inn Tower Bridge [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		0.00%	[1],[3]				50.00%
Investment in Real Estate Ventures, at equity		$ 0	[3]	723	[3]		$ 900
Company’s share of income (loss) (Company’s basis)	[3]	367		107
Equity Method Investment Summarized Financial Information Debt	[3]	$ 0		13,394
Debt Instrument, Interest Rate, Effective Percentage	[3]	0.00%
Debt Instrument, Interest Rate Terms	[3]	0
Brandywine Nineteen Nineteen Ventures [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		50.00%	[1],[2],[5]						50.00%
Investment in Real Estate Ventures, at equity	[2],[5]	$ 29,086		12,823
Company’s share of income (loss) (Company’s basis)	[2],[5]	0		(21)
Equity Method Investment Summarized Financial Information Debt	[2],[5]	$ 19,411		0
Debt Instrument, Interest Rate Terms	[2],[5]	0
51 N 50 Patterson [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		70.00%	[1]					70.00%
Investment in Real Estate Ventures, at equity		$ 16,725		0
Company’s share of income (loss) (Company’s basis)		0		0
Equity Method Investment Summarized Financial Information Debt		$ 0		0
Debt Instrument, Interest Rate, Effective Percentage		0.00%
1250 First Street Office [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage		70.00%	[1]					70.00%
Investment in Real Estate Ventures, at equity		$ 14,312		0
Company’s share of income (loss) (Company’s basis)		0		0
Equity Method Investment Summarized Financial Information Debt		$ 0		0
Debt Instrument, Interest Rate, Effective Percentage		0.00%
Seven Tower Bridge [Member]
Schedule Of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	[1]	20.00%
Investment in Real Estate Ventures, at equity		$ 491		315
Company’s share of income (loss) (Company’s basis)		(135)		(388)
Equity Method Investment Summarized Financial Information Debt		$ 14,789		$ 14,865
Debt Instrument, Interest Rate, Effective Percentage		3.44%

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

[2] 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).

[3] The ownership interest in Invesco, L.P. was sold prior to December 31, 2014. The Company purchased the remaining 50% interest in Broadmoor Austin Associates on June 22, 2015. The ownership interest in DRA-N. PA was disposed of on August 12, 2015. The ownership interest in Residence Inn Tower Bridge was sold on December 30, 2015. See below for further detail on 2015 dispositions.

[4] Carrying amount represents the negative investment balance of the venture and is included in other liabilities.

[5] The stated rate for the construction loan is LIBOR + 2.25%. The interest rate spread decreases to 2.00% upon receipt of certificate of occupancy. It is further reduced to 1.75% upon stabilization. To fulfill interest rate protection requirements an interest rate cap was purchased at 4.50%.