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Investment in Unconsolidated Ventures Company's Investment in Real Estate Ventures and the Company's Share of the Real Estate Ventures' Income (Loss) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014 acre	Dec. 31, 2013	Dec. 31, 2012	Jul. 31, 2013 acre	Jul. 31, 2014	Mar. 26, 2013	Jan. 20, 2011 acre	Jun. 19, 2013	Apr. 03, 2014
Schedule of Equity Method Investments [Line Items]
Area Owned By Real Estate Venture Of Undeveloped Parcels Of Land	3.0
Equity Method Investments	$ 225,004,000us-gaap_EquityMethodInvestments	$ 180,512,000us-gaap_EquityMethodInvestments	$ 193,555,000us-gaap_EquityMethodInvestments
Income (Loss) from Equity Method Investments	(790,000)	3,664,000	2,741,000
Equity Method Investment Summarized Financial Information Debt	965,077,000
Equity Method Investments including net liabilities	223,790,000
Equity Method Investments [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment Summarized Financial Information Debt	965,077,000
Broadmoor Austin Associates [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
Equity Method Investments	65,407,000
Income (Loss) from Equity Method Investments	(460,000)
Equity Method Investment Summarized Financial Information Debt	53,516,000
Debt Instrument, Interest Rate, Effective Percentage	7.04%
Debt Instrument, Maturity Date	Apr. 10, 2023
HSRE, LLC [Member] [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	30.00%
Equity Method Investments	14,314,000
Income (Loss) from Equity Method Investments	67,000
Equity Method Investment Summarized Financial Information Debt	90,245,000
Debt Instrument, Interest Rate Terms	L+2.20%
Debt Instrument, Maturity Date	Jul. 25, 2016
4040 Wilson Venture [Member]
Schedule of Equity Method Investments [Line Items]
Area Owned By Real Estate Venture Of Undeveloped Parcels Of Land				1.3
Equity Method Investment, Ownership Percentage	50.00%
Equity Method Investments	27,415,000
Income (Loss) from Equity Method Investments	(132,000)
Equity Method Investment Summarized Financial Information Debt	0
Brandywine - Al Venture [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
Equity Method Investments	45,712,000
Income (Loss) from Equity Method Investments	(185,000)
Equity Method Investment Summarized Financial Information Debt	133,843,000
Debt Instrument, Interest Rate, Effective Percentage	3.93%
Austin Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
Equity Method Investments	40,374,000
Income (Loss) from Equity Method Investments	(574,000)
Equity Method Investment Summarized Financial Information Debt	382,100,000				34,500,000
Debt Instrument, Interest Rate, Effective Percentage	3.57%
Equity method investments, Closing costs	200,000				100,000
BDN Beacon Venture LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage						20.00%
Equity Method Investments						17,000,000
TB-BDN Plymouth Apartments [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
Equity Method Investments	12,720,000
Income (Loss) from Equity Method Investments	(83,000)
Equity Method Investment Summarized Financial Information Debt	29,481,000
Debt Instrument, Interest Rate Terms	L+1.70%
Debt Instrument, Maturity Date	Dec. 20, 2017
Brandywine Nineteen Nineteen Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Area Owned By Real Estate Venture Of Undeveloped Parcels Of Land							1
Equity Method Investment, Ownership Percentage	50.00%						50.00%
Equity Method Investments	12,823,000
Income (Loss) from Equity Method Investments	(21,000)
Equity Method Investment Summarized Financial Information Debt	0
Debt Instrument, Interest Rate Terms	L+2.25%
Debt Instrument, Maturity Date	Oct. 21, 2018
Debt Instrument, Interest Rate Terms	LIBOR + 2.25%
Interest rate upon receipt of certificate of occupancy	2.00%
Interest rate upon stabilization	1.75%
Interest rate protection cap	4.50%
One Thousand Chesterbrook Blvd. [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
Equity Method Investments	2,152,000
Income (Loss) from Equity Method Investments	388,000
Equity Method Investment Summarized Financial Information Debt	24,251,000
Debt Instrument, Interest Rate, Effective Percentage	4.75%
Debt Instrument, Maturity Date	Dec. 01, 2021
Four Tower Bridge [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	65.00%
Equity Method Investments	1,474,000
Income (Loss) from Equity Method Investments	(144,000)
Equity Method Investment Summarized Financial Information Debt	10,353,000
Debt Instrument, Interest Rate, Effective Percentage	5.20%
Debt Instrument, Maturity Date	Feb. 10, 2021
Residence Inn Tower Bridge [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
Equity Method Investments	723,000
Income (Loss) from Equity Method Investments	107,000
Equity Method Investment Summarized Financial Information Debt	13,394,000
Debt Instrument, Interest Rate, Effective Percentage	5.63%
Debt Instrument, Maturity Date	Feb. 11, 2016
PJP VII [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	25.00%
Equity Method Investments	911,000
Income (Loss) from Equity Method Investments	177,000
Equity Method Investment Summarized Financial Information Debt	5,785,000
Debt Instrument, Interest Rate Terms	L+2.65%
Debt Instrument, Maturity Date	Dec. 31, 2019
PJP Two [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	30.00%
Equity Method Investments	403,000
Income (Loss) from Equity Method Investments	73,000
Equity Method Investment Summarized Financial Information Debt	3,492,000
Debt Instrument, Interest Rate, Effective Percentage	6.12%
Debt Instrument, Maturity Date	Nov. 01, 2023
G&I Interchange Office LLC (DRA — N. PA) [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	20.00%
Equity Method Investments	0
Income (Loss) from Equity Method Investments	(658,000)
Equity Method Investment Summarized Financial Information Debt	174,595,000
Debt Instrument, Interest Rate, Effective Percentage	5.78%
Debt Instrument, Maturity Date	Feb. 28, 2015
Sale of ownership in properties	80.00%
Seven Tower Bridge [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	20.00%
Equity Method Investments	315,000
Income (Loss) from Equity Method Investments	(388,000)
Equity Method Investment Summarized Financial Information Debt	14,865,000
Debt Instrument, Interest Rate, Effective Percentage	3.78%
PJP Five [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	25.00%
Equity Method Investments	216,000
Income (Loss) from Equity Method Investments	166,000
Equity Method Investment Summarized Financial Information Debt	5,242,000
Debt Instrument, Interest Rate, Effective Percentage	6.47%
Debt Instrument, Maturity Date	Aug. 11, 2019
Two Tower Bridge [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments								(100,000)
PJP Six [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	25.00%
Equity Method Investments	45,000
Income (Loss) from Equity Method Investments	(67,000)
Equity Method Investment Summarized Financial Information Debt	8,168,000
Debt Instrument, Interest Rate, Effective Percentage	6.08%
Debt Instrument, Maturity Date	Apr. 01, 2023
Coppell Associates [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
Equity Method Investments	(1,214,000)
Income (Loss) from Equity Method Investments	(45,000)
Equity Method Investment Summarized Financial Information Debt	15,747,000
Debt Instrument, Interest Rate, Effective Percentage	5.75%
Debt Instrument, Maturity Date	Feb. 01, 2016
Invesco, L.P. [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	0
Income (Loss) from Equity Method Investments	989,000
Secured Debt [Member]
Schedule of Equity Method Investments [Line Items]
Long-term Debt, Weighted Average Interest Rate	5.72%	5.73%	6.65%
Secured Debt [Member] \| Brandywine - Al Venture [Member]
Schedule of Equity Method Investments [Line Items]
Long-term Debt, Weighted Average Interest Rate	3.93%
Secured Debt [Member] \| Austin Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Long-term Debt, Weighted Average Interest Rate	3.57%
Secured Debt [Member] \| Seven Tower Bridge [Member]
Schedule of Equity Method Investments [Line Items]
Long-term Debt, Weighted Average Interest Rate	3.78%
Secured Debt [Member] \| Fifth mortgage - swapped to fixed [Member] \| Austin Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Face Amount	34,500,000
Debt Instrument, Interest Rate, Stated Percentage	3.87%
Secured Debt [Member] \| Fixed rate loan maturing January 2019 [Member] \| Brandywine - Al Venture [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Face Amount	39,300,000
Debt Instrument, Interest Rate, Stated Percentage	4.40%
Secured Debt [Member] \| Fixed rate loan maturing January 2022 [Member] \| Brandywine - Al Venture [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Face Amount	28,000,000
Debt Instrument, Interest Rate, Stated Percentage	4.65%
Secured Debt [Member] \| Fixed rate loan maturing August 2019 [Member] \| Brandywine - Al Venture [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Face Amount	66,500,000
Debt Instrument, Interest Rate, Stated Percentage	3.22%
Secured Debt [Member] \| First mortgage - swapped to fixed [Member] \| Austin Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Face Amount	34,000,000
Derivative, Fixed Interest Rate	1.59%
Derivative, Basis Spread on Variable Rate	1.93%
Debt Instrument, Interest Rate, Effective Percentage	3.52%
Secured Debt [Member] \| Second mortgage - swapped to fixed [Member] \| Austin Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Face Amount	56,000,000
Derivative, Fixed Interest Rate	1.49%
Derivative, Basis Spread on Variable Rate	1.70%
Debt Instrument, Interest Rate, Effective Percentage	3.19%
Secured Debt [Member] \| Third mortgage - swapped to fixed [Member] \| Austin Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Face Amount	140,600,000
Derivative, Fixed Interest Rate	1.43%
Derivative, Basis Spread on Variable Rate	2.01%
Debt Instrument, Interest Rate, Effective Percentage	3.44%
Secured Debt [Member] \| Fourth mortgage - swapped to fixed [Member] \| Austin Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Face Amount	29,000,000
Debt Instrument, Interest Rate, Stated Percentage	4.50%
Secured Debt [Member] \| Mortgage Maturing February 2015 [Domain] \| Brandywine - Al Venture [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Number of Fixed Rate Mortgages	3
Secured Debt [Member] \| Mortgage Maturing February 2015 [Domain] \| Seven Tower Bridge [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment Summarized Financial Information Debt	8,000,000
Debt Instrument, Number of Fixed Rate Mortgages	2
Debt Instrument, Interest Rate, Stated Percentage	7.00%
Secured Debt [Member] \| Fixed Rate Loan Maturing September 2025 [Member] \| Seven Tower Bridge [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment Summarized Financial Information Debt	1,000,000
Debt Instrument, Interest Rate, Stated Percentage	3.00%
Secured Debt [Member] \| Fixed Rate Loan Maturing February 2016 [Member] \| Seven Tower Bridge [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment Summarized Financial Information Debt	2,000,000
Debt Instrument, Interest Rate, Stated Percentage	4.00%
Secured Debt [Member] \| Fixed Rate Loan Maturing March 2020 [Member] \| Seven Tower Bridge [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment Summarized Financial Information Debt	3,900,000
Debt Instrument, Interest Rate, Stated Percentage	0.00%
Four Points Centre [Member] \| Austin Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Non-Recourse Debt									29,000,000
Secured Debt [Member] \| Austin Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Face Amount	$ 88,000,000
Debt Instrument, Interest Rate, Stated Percentage	4.00%