Investment in Unconsolidated Ventures (Narrative) (Details) (USD $)	12 Months Ended				0 Months Ended
	Dec. 31, 2014 sqft Buildings Real_Estate_Ventures Room acre ApartmentUnits	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Oct. 17, 2014	Jul. 31, 2014	Jul. 31, 2013 acre sqft	Jun. 19, 2013	Oct. 16, 2013	Apr. 03, 2014	Jan. 20, 2011 acre	Mar. 26, 2013	Jan. 25, 2013 beds Storey acre	Oct. 20, 2014	Oct. 21, 2014	Dec. 19, 2013 Storey
Schedule of Equity Method Investments [Line Items]
Number of apartment units in a Real Estate Venture	345
Net gain (loss) on real estate venture transactions	$ (417,000)	$ 29,604,000	$ (950,000)
Net proceeds from the contribution of land to an unconsolidated real estate venture	8,212,000	0	0
Payments to Acquire Equity Method Investments	46,098,000	33,069,000	65,354,000
Company's share of equity (Company's basis)	225,004,000	180,512,000	193,555,000
Net gain on real estate venture transactions	(417,000)	3,683,000	0
Number of unconsolidated Real Estate Ventures	17
Equity Method Investments including net liabilities	223,790,000
Number of real estate ventures that owns 51 office buildings	11
Number of office buildings owned by 15 Real Estate Ventures	65
Net rentable square feet of 51 office buildings owned the 15 Real Estate Ventures	6,700,000
Number of real estate ventures that owns undeveloped land	1
Area of undeveloped parcels of land (in acres) owned by a Real Estate Venture	3.0
Number of real estate ventures that owns land under development	4
Area Owned By Real Estate Venture in Parcels Of Land Under Development	22.6
Number of real estate ventures developed hotel property	1
Number of rooms in a hotel property developed by one Real Estate Venture	137
Minimum ownership interest in unconsolidated Real Estate Ventures	20.00%
Maximum ownership interest in unconsolidated Real Estate Ventures	65.00%
Net gain (loss) on sale of undepreciated real estate	458,000	6,866,000	0
Brandywine Nineteen Nineteen Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	50.00%										50.00%
Company's share of equity (Company's basis)	12,823,000
Area of undeveloped parcels of land (in acres) owned by a Real Estate Venture											1
Payments to Acquire Land				9,300,000
Austin, Texas [Member]
Schedule of Equity Method Investments [Line Items]
Number of floors of a building						3
BDN Beacon Venture LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage												20.00%
Company's share of equity (Company's basis)												17,000,000
Austin Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, Consideration transferred					128,100,000	48,200,000
Short-term Bank Loans and Notes Payable	88,000,000
Number of Real Estate Properties					7	2
Potential future capital contributions under joint ventures which are not contractually binding	100,000,000
Equity method investments, Cash contributions	69,000,000					12,800,000
Equity method investments, Closing costs	200,000					100,000
Equity Method Investment, Ownership Percentage	50.00%
Company's share of equity (Company's basis)	40,374,000
Two Tower Bridge [Member]
Schedule of Equity Method Investments [Line Items]
Company's share of equity (Company's basis)								(100,000)
Net gain on real estate venture transactions		3,683,000
Investments, Fair Value Disclosure								3,600,000
4040 Wilson Venture [Member]
Schedule of Equity Method Investments [Line Items]
Area of Real Estate Property							426,900
Equity Method Investment, Ownership Percentage	50.00%
Equity Method Investment, Budgeted Total Project Cost							194,600,000
Payments to Acquire Equity Method Investments							72,000,000
Guaranty Percentage	100.00%
Company's share of equity (Company's basis)	27,415,000
Area of undeveloped parcels of land (in acres) owned by a Real Estate Venture							1.3
Six Tower Bridge [Member]
Schedule of Equity Method Investments [Line Items]
Net gain (loss) on sale of undepreciated real estate		7,800,000
Business Combination, Step Acquisition, Equity Interest in Acquiree, Fair Value								24,500,000
Business Acquisition Purchase Price Allocation Building								14,800,000
Business Acquisition Purchase Price Allocation, Land Portion								6,900,000
Business Acquisition Purchase Price Allocation Intangible Assets								3,300,000
Business Acquisition Purchase Price Allocated To Market Lease Liabilities								500,000
The Grove at Cira Centre South [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments, Cash contributions	0
Number of floors of a building													33
Student housing bed capacity													850
Equity Method Investment, Budgeted Total Project Cost	158,500,000
Payments to Acquire Equity Method Investments	60,700,000
Construction Loan	97,800,000
Equity Method Investment, Aggregate Cost													4,000,000
Area of undeveloped parcels of land (in acres) owned by a Real Estate Venture													1
Equity Method Investment, Underlying Equity in Net Assets													8,500,000
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity													4,500,000
TB-BDN Plymouth Apartments [Member]
Schedule of Equity Method Investments [Line Items]
Guaranty Liabilities	3,200,000
PJP VII [Member]
Schedule of Equity Method Investments [Line Items]
Guaranty Liabilities	500,000
The Grove at Cira Centre South [Member]
Schedule of Equity Method Investments [Line Items]
Guaranty Liabilities	24,700,000
Third Parties and Real Estate Ventures [Member] | Brandywine Nineteen Nineteen Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage														50.00%
LCOR/Calstrs [Member] | Brandywine Nineteen Nineteen Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Aggregate gross sales price of a joint venture															16,400,000
General Partners' Contributed Capital															5,200,000
Equity method investments, Cash contributions	13,400,000
Net proceeds from the contribution of land to an unconsolidated real estate venture	8,200,000
Ashton Park [Member] | 4040 Wilson Venture [Member]
Schedule of Equity Method Investments [Line Items]
Fair value of land, net of amount encumbered							36,000,000
Brandywine Realty Trust [Member] | Austin, Texas [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, agreement to pay future capital expenditures	4,400,000
Brandywine Realty Trust [Member] | Austin Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Number of properties contributed to a joint venture									7
Area of Real Estate Property									1,398,826
Aggregate gross sales price of a joint venture									330,000,000
Equity method investment, agreement to pay future capital expenditures		800,000							5,200,000
Equity method investments, Cash contributions					18,900,000	6,400,000
Net gain (loss) on real estate venture transactions		25,900,000
Equity Method Investment, Ownership Percentage	50.00%
Equity Method Investment, Additional Distribution Rights	10.00%
Brandywine Realty Trust [Member] | 4040 Wilson Venture [Member]
Schedule of Equity Method Investments [Line Items]
Payments to Acquire Interest in Joint Venture	26,800,000						36,000,000
DRA Advisors LLC [Member] | Austin Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Distribution, Net of Initial Funding Agreement									266,300,000
Amount Of Participant Contribution									49,700,000
Equity method investments, Closing costs									1,900,000
Closing Prorations and Lender Holdbacks									6,900,000
Proceeds from Equity Method Investment, Dividends or Distributions									271,500,000
Equity Method Investment, Ownership Percentage	50.00%
Non-Recourse Debt									230,600,000
Campus Crest Properties, LLC [Member] | The Grove at Cira Centre South [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage													30.00%
HSRE, LLC [Member] [Member] | The Grove at Cira Centre South [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage													40.00%
Four Points Centre [Member]
Schedule of Equity Method Investments [Line Items]
Number of Real Estate Properties										2
Number of floors of a building										3						3
Four Points Centre [Member] | Austin Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Non-Recourse Debt										29,000,000
Four Points Centre [Member] | Brandywine Realty Trust [Member] | Austin Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Aggregate gross sales price of a joint venture										41,500,000
Amount Of Participant Contribution										5,900,000
Proceeds from Equity Method Investment, Dividends or Distributions										34,400,000
Four Points Centre [Member] | DRA Advisors LLC [Member] | Austin Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Amount Of Participant Contribution										$ 5,900,000