Investment in Unconsolidated Ventures Investment in Unconsolidated Ventures (Narrative) (Details) (USD $)	3 Months Ended		6 Months Ended				6 Months Ended			6 Months Ended
	Jun. 30, 2013 Buildings Real_Estate_Ventures sqft Room acre	Jun. 30, 2012	Jun. 30, 2013 Buildings Real_Estate_Ventures sqft Room acre	Jun. 30, 2012	Dec. 31, 2012	Jun. 19, 2013 Two Tower Bridge [Member]	Jun. 30, 2013 Six Tower Bridge [Member]	Jun. 19, 2013 Six Tower Bridge [Member]	Mar. 26, 2013 BDN Beacon Venture LLC [Member]	Jun. 30, 2013 The Grove at Cira Centre South [Member]	Jan. 25, 2013 The Grove at Cira Centre South [Member] beds acre Storey	Jan. 25, 2013 The Grove at Cira Centre South [Member] Brandywine Realty Trust [Member]	Jan. 25, 2013 The Grove at Cira Centre South [Member] Campus Crest Properties, LLC [Member]	Jan. 25, 2013 The Grove at Cira Centre South [Member] HSRE, LLC [Member] [Member]
Schedule of Equity Method Investments [Line Items]
Gain from remeasurement of investment in a real estate venture	$ 7,847,000	$ 0	$ 7,847,000	$ 0			$ 7,800,000
Number of floors of a building											33
Student housing bed capacity											850
Company's share of equity (Company's Basis)	176,875,000		176,875,000		193,555,000	(100,000)			17,000,000
Net gain (loss) on real estate venture transactions	3,683,000		3,683,000	0
Number of unconsolidated investments in Real Estate Ventures	17		17
Number of real estate ventures that owns buildings	12		12
Number of office buildings in Real Estate Ventures	50		50
Aggregate Approximate Area Of Office Buildings	6,200,000		6,200,000
Number of real estate ventures that owns undeveloped land	3		3
Area Owned By Real Estate Venture Of Undeveloped Parcels Of Land	24		24								1
Equity Method Investment, Underlying Equity in Net Assets											8,500,000
Equity Method Investment, Budgeted Total Project Cost										158,500,000
Payments to Acquire Equity Method Investments			12,568,000	18,617,000						60,700,000
Construction Loan										97,800,000
Equity Method Investment, Aggregate Cost											6,000,000
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity											2,500,000
Number of real estate ventures that owns land under development	1		1
Area Owned By Real Estate Venture in Parcels Of Land Under Development	1		1
Number of real estate ventures developed hotel property	1		1
Number Of Rooms Contain In One Real Estate Venture Developed As Hotel Property	137		137
Minimum Percentage Of Unconsolidated Equity Method Investment Ownership			20.00%
Maximum Percentage Of Unconsolidated Equity Method Investment Ownership			65.00%
Guarantor Obligations, Current Carrying Value	600,000		600,000
Guarantor Obligations, Maximum Exposure, Undiscounted										23,000,000
Equity Method Investment, Ownership Percentage									20.00%			30.00%	30.00%	40.00%
Business Acquisition, Purchase Price Allocation, Assets Acquired (Liabilities Assumed), Net								24,500,000
Business Acquisition Purchase Price Allocation Building								14,800,000
Business Acquisition, Purchase Price Allocation, Land								6,900,000
Business Acquisition Purchase Price Allocation Intangible Assets								3,300,000
Business Acquisition Purchase Price Allocated To Market Lease Liabilities								500,000
Investments, Fair Value Disclosure						$ 3,600,000