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 $) In Thousands, unless otherwise specified	12 Months Ended				12 Months Ended					12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Nov. 30, 2010	Dec. 31, 2011 Broadmoor Austin Associates [Member]		Dec. 31, 2011 Brandywine - Al Venture [Member]		Dec. 20, 2011 Brandywine - Al Venture [Member]	Dec. 31, 2011 Macquarie BDN Christina LLC [Member]		Dec. 31, 2011 One Commerce Square [Member]		Dec. 31, 2011 Brandywine 1919 Ventures [Member]		Dec. 31, 2011 Two Commerce Square [Member]		Dec. 31, 2011 1000 Chesterbrook Blvd. [Member]		Dec. 31, 2011 Four Tower Bridge [Member]		Dec. 31, 2011 G&I Interchange Office LLC (DRA — N. PA) [Member]		Dec. 31, 2011 Residence Inn Tower Bridge [Member]		Dec. 31, 2011 PJP VII [Member]		Dec. 31, 2011 PJP V [Member]		Dec. 31, 2011 PJP II [Member]		Dec. 31, 2011 Six Tower Bridge [Member]		Dec. 31, 2011 Two Tower Bridge [Member]		Dec. 31, 2011 Seven Tower Bridge [Member]		Dec. 31, 2011 PJP VI [Member]		Dec. 31, 2011 Eight Tower Bridge [Member]		Dec. 31, 2011 Coppell Associates [Member]		Dec. 31, 2011 Five Tower Bridge [Member]	Dec. 31, 2011 Invesco, L.P. [Member]
Schedule of Equity Method Investments [Line Items]
Company's ownership percentage in the Real Estate Venture				25.00%	50.00%	[1]	50.00%	[1],[2]	50.00%	20.00%	[1]	25.00%	[1],[3]	50.00%	[1],[4]	25.00%	[1],[3]	50.00%	[1]	65.00%	[1],[5]	20.00%	[1],[6]	50.00%	[1]	25.00%	[1]	25.00%	[1]	30.00%	[1]	63.00%	[1],[5]	35.00%	[1],[7]	20.00%	[1],[8]	25.00%	[1]	3.40%	[1]	50.00%	[1],[5],[9]
Company's share of equity (Company's basis)	$ 115,807	$ 84,372			$ 66,150		$ 24,670	[2]		$ 6,168		$ 5,100	[3]	$ 4,882	[4]	$ 3,415	[3]	$ 1,745		$ 1,360	[5]	$ 1,239	[6]	$ 481		$ 472		$ 361		$ 300		$ 201	[5]	$ 134	[7]	$ 105	[8]	$ 31		$ 0		$ (1,007)	[5],[9]	$ 0	$ 0
Company's share of income (Company's basis)	3,775	5,305	4,069		197		(278)	[2]		875		242	[3]	0	[4]	224	[3]	245		702	[5]	76	[6]	433		190		116		138		126	[5]	(364)	[7]	0	[8]	67		583		(139)	[5],[9]	60	282
Real Estate Venture Debt at 100%	$ 745,830				$ 66,810		$ 68,000	[2]		$ 60,000		$ 128,529	[3]	$ 0	[4]	$ 107,112	[3]	$ 26,000		$ 10,870	[5]	$ 181,967	[6]	$ 14,261		$ 8,072		$ 5,789		$ 4,263		$ 12,599	[5]	$ 13,482	[7]	$ 11,182	[8]	$ 8,834		$ 0		$ 18,060	[5],[9]
Debt Instrument Interest Rates Effective Percentage	LIBOR plus 0.725	LIBOR plus 0.725																								L+1.55%
Current Interest Rate					7.04%					4.50%		5.67%	[3]			6.30%	[3]	4.75%		5.20%	[5]	5.78%	[6]	5.63%				6.47%		6.12%		7.79%	[5]					6.08%

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

[2] See Note 3 for related discussion about the Brandywine-AI Venture, LLC formation.

[3]

[4] See Note 3 for related discussion about the Brandywine 1919 Market Ventures formation.

[5] As a result of the adoption of the new accounting standard for the consolidation of variable interest entities, the Company deconsolidated these three real estate ventures as of January 1, 2010.

[6] Proceeds received by the Company from the sale of an 80% ownership stake in the properties exceeded the historical cost of those properties. Accordingly, no investment in the real estate venture was reflected on the balance sheet at period end.

[7] Consists of a $10.1 million fixed rate mortgage with a 5.72% interest rate and May 2013 maturity date; also includes $3.4 million municipal and state borrowings with nominal interest rates (0 — 1%) and maturity dates from 3 to 10 years.

[8] Comprised of two fixed rate mortgages: $7.0 million and $1.0 million. Both mature in February 2013 and accrue interest at a fixed rate of 4% through February 2012, and 5% through February 2013. The total loan amount of $11.2 million as presented above also includes a 3% fixed rate loan (interest only through maturity) with a balance of $1.2 million as of December 31, 2011. This loan can be increased up to $1.3 million through its maturity date of September 2025. The remaining $2.0 million of the total amount features a 4% fixed rate with interest only through its maturity date of February 2014.

[9] Comprised of two fixed rate mortgages: a senior fixed rate note of $16.4 million at 5.75% that matures in March 2016 and a junior fixed rate note of $1.6 million at 6.89% that matures in December 2013; 5.89% is the blended rate.