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								12 Months Ended						12 Months Ended																					12 Months Ended																											12 Months Ended
	Dec. 31, 2013 acre	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Broadmoor Austin Associates [Member]		Dec. 31, 2013 HSRE, LLC [Member] [Member]	Dec. 31, 2013 4040 Wilson Venture [Member]	Jul. 31, 2013 4040 Wilson Venture [Member] acre	Dec. 31, 2013 Brandywine - Al Venture [Member]		Dec. 31, 2013 Austin Joint Venture [Member]		Dec. 31, 2013 BDN Beacon Venture LLC [Member]	Mar. 26, 2013 BDN Beacon Venture LLC [Member]	Dec. 31, 2013 TB-BDN Plymouth Apartments [Member]	Dec. 31, 2013 Six Tower Bridge [Member]		Dec. 31, 2013 One Commerce Square [Member]		Dec. 31, 2013 Two Commerce Square [Member]		Dec. 31, 2013 Brandywine Nineteen Nineteen Ventures [Member]	Dec. 31, 2013 One Thousand Chesterbrook Blvd. [Member]	Dec. 31, 2013 Four Tower Bridge [Member]	Dec. 31, 2013 Residence Inn Tower Bridge [Member]	Dec. 31, 2013 PJP VII [Member]	Dec. 31, 2013 PJP Two [Member]	Dec. 31, 2013 G&I Interchange Office LLC (DRA — N. PA) [Member] Rate		Dec. 31, 2013 Seven Tower Bridge [Member]		Dec. 31, 2013 PJP Five [Member]	Dec. 31, 2013 Two Tower Bridge [Member]		Jun. 19, 2013 Two Tower Bridge [Member]	Dec. 31, 2013 PJP Six [Member]	Dec. 31, 2013 Coppell Associates [Member]	Dec. 31, 2013 Eight Tower Bridge [Member]	Dec. 31, 2013 Invesco, L.P. [Member]	Dec. 31, 2013 Secured Debt [Member]	Dec. 31, 2012 Secured Debt [Member]	Dec. 31, 2011 Secured Debt [Member]	Dec. 31, 2013 Secured Debt [Member] Brandywine - Al Venture [Member]		Dec. 31, 2013 Secured Debt [Member] Austin Joint Venture [Member]		Dec. 31, 2013 Secured Debt [Member] Seven Tower Bridge [Member]		Dec. 31, 2013 Secured Debt [Member] Fixed rate loan maturing January 2019 [Member] Brandywine - Al Venture [Member]		Dec. 31, 2013 Secured Debt [Member] Fixed rate loan maturing January 2022 [Member] Brandywine - Al Venture [Member]		Dec. 31, 2013 Secured Debt [Member] Fixed rate loan maturing August 2019 [Member] Brandywine - Al Venture [Member]		Dec. 31, 2013 Secured Debt [Member] First mortgage - swapped to fixed [Member] Austin Joint Venture [Member]		Dec. 31, 2013 Secured Debt [Member] Second mortgage - swapped to fixed [Member] Austin Joint Venture [Member]		Dec. 31, 2013 Secured Debt [Member] Third mortgage - swapped to fixed [Member] Austin Joint Venture [Member]		Dec. 31, 2013 Secured Debt [Member] Mortgage Maturing February 2015 [Domain] Brandywine - Al Venture [Member] Fixed_Rate_Mortgages		Dec. 31, 2013 Secured Debt [Member] Mortgage Maturing February 2015 [Domain] Seven Tower Bridge [Member] Fixed_Rate_Mortgages		Dec. 31, 2013 Secured Debt [Member] Fixed Rate Loan Maturing September 2025 [Member] Seven Tower Bridge [Member]		Dec. 31, 2013 Secured Debt [Member] Fixed Rate Loan Maturing February 2014 [Member] Seven Tower Bridge [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Face Amount																																																	$ 40,000,000	[1]	$ 28,000,000	[2]	$ 66,500,000	[1]	$ 34,000,000	[3]	$ 56,000,000	[3]	$ 140,600,000	[3]
Long-term Debt, Weighted Average Interest Rate																																								5.73%	6.65%	6.72%	3.92%	[1]	3.39%	[3]	4.22%	[4]
Area Owned By Real Estate Venture Of Undeveloped Parcels Of Land	3.8							1.3
Equity Method Investment, Ownership Percentage				50.00%	[5]	30.00%	50.00%	50.00%	50.00%		50.00%		20.00%	20.00%	50.00%	63.00%	[2]	25.00%	[4]	25.00%	[6]	50.00%	50.00%	65.00%	50.00%	25.00%	30.00%	20.00%	[4],[5]	20.00%		25.00%	35.00%	[7]		25.00%	50.00%
Equity Method Investments	180,512,000	193,555,000	115,807,000	65,867,000		13,761,000	13,499,000		46,406,000		17,262,000		0	17,000,000	12,402,000	0		0		0		6,214,000	2,064,000	1,617,000	742,000	783,000	330,000	0		447,000		175,000	0		(100,000)	112,000	(1,169,000)	0	0
Income (Loss) from Equity Method Investments	3,664,000	2,741,000	3,775,000	(52,000)		0	0		130,000		(434,000)		907,000		5,000	165,000		933,000		614,000		0	461,000	320,000	326,000	239,000	(87,000)	(619,000)		0		158,000	265,000			70,000	(78,000)	3,000	338,000
Equity Method Investment Summarized Financial Information Debt	699,860,000			56,616,000		966,000	0		134,500,000		230,600,000		0		0	0		0		0		0	24,861,000	10,535,000	13,700,000	6,250,000	3,765,000	177,207,000		11,035,000		5,437,000	0			8,404,000	15,984,000																										8,000,000	[4]	1,000,000	[4]	2,000,000	[4]
Debt Instrument, Interest Rate Terms						L+2.20%									L+1.70%											L+2.65%
Debt Instrument, Interest Rate, Effective Percentage				7.04%					3.92%		3.39%												4.75%	5.20%	5.63%		6.12%	5.78%		4.22%		6.47%				6.08%	5.75%
Debt Instrument Maturity Dates				Apr-23		Jul-16			(b)	[1]	(c)	[3]			Dec-17								Dec-21	Feb-21	Feb-16	Dec-19	Nov-23	Jan-15		(d)	[8]	Aug-19				Apr-23	Feb-16
Sale of ownership in properties																												80.00%	[4]
Debt Instrument, Number of Fixed Rate Mortgages																																																													3	[4]	2	[4]
Debt Instrument, Interest Rate, Stated Percentage																																																	4.40%	[1]	4.65%	[1]	3.22%	[1]	3.52%	[3]	3.19%	[3]	3.44%	[3]			6.00%	[4]	3.00%	[4]	4.00%	[4]
Debt Instrument, Interest Rate Increase																																																															1.00%	[4]
Proceeds from Equity Method Investment, Dividends or Distributions	$ 7,496,000	$ 5,109,000	$ 10,316,000

[1] (b)The debt for these properties is comprised of three fixed rate mortgages: (1) $40.0 million with a 4.40% fixed interest rate due January 1, 2019, (2) $28.0 million with a 4.65% fixed interest rate due January 1, 2022, and (3) $66.5 million with a 3.22% fixed interest rate due August 1, 2019, resulting in a time and dollar weighted average rate of 3.92%.

[2] (g)On June 19, 2013, we acquired ownership of Six Tower Bridge. Real estate venture income reflects our previous partnership interest for the year to date period through June 18, 2013.

[3] (c)The debt for these properties is comprised of three mortgages: (1) $34.0 million with a 3.52% interest rate due November 1, 2018, (2) $56.0 million with a 3.19% interest rate due October 15, 2018, and (3) $140.6 million with a rate of 3.44% fixed interest rate due November 1, 2018, resulting in a time and dollar weighted average rate of 3.39%.

[4] (e)Proceeds received by the Company from the sale of an 80% ownership stake in the properties exceeded the historical cost of those properties. No investment in the real estate venture was reflected on the balance sheet at formation.

[5] (f)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).

[6] (h)On December 19, 2013, we increased our ownership interest in One and Two Commerce Square. The real estate venture income reflects our previous partnership interest for the year to date period through December 18, 2013.

[7] (i)The ownership interest in these properties was disposed of prior to December 31, 2013. For further information on BDN Beacon LLC, see the section entitled BDN Beacon Venture below. For further information on Two Tower Bridge, see the section entitled Two and Six Tower Bridge Exchange Transaction below.

[8] (d)Comprised of two fixed rate mortgages totaling $8.0 million that mature on February 8, 2015 and accrue interest at a current rate of 6% (increasing by 1% annually through maturity), a $1.0 million 3% fixed rate loan with interest only through its September 1, 2025 maturity, and a $2.0 million 4% fixed rate loan with interest only through its February 7, 2016 maturity, resulting in a time and dollar weighted average rate of 4.22%.