Investment in Unconsolidated Real Estate Ventures (Textual) (Details)																								3 Months Ended			9 Months Ended		12 Months Ended
		Dec. 28, 2018 USD ($) ft² property	Dec. 20, 2018 USD ($) ft² property	Dec. 11, 2018 USD ($) ft² property	Aug. 01, 2018 USD ($)	Jun. 26, 2018 USD ($)	Jan. 10, 2018 USD ($) property	Jan. 05, 2018 USD ($) ft²	Oct. 18, 2017 USD ($) ft² property	Oct. 17, 2017 USD ($) property	Sep. 14, 2017 USD ($) ft² property	Jan. 31, 2017 USD ($) property	Jun. 10, 2016 USD ($)	Mar. 02, 2016 USD ($)	Feb. 04, 2016 USD ($) ft² property	May 29, 2015 USD ($) a	Jan. 30, 2015 USD ($)	Oct. 27, 2014 ft² Apartment_unit Storey NumberofCar.	Oct. 21, 2014 USD ($)	Oct. 17, 2014 USD ($) ft² property	Jul. 31, 2014 USD ($) ft² property Storey	Oct. 16, 2013 USD ($) ft² property	Jul. 31, 2013 USD ($) ft² a property	Dec. 31, 2018 USD ($) ft² a property Real_Estate_Investment Apartment_unit		Sep. 30, 2018 USD ($) property	Sep. 30, 2017 USD ($) property	Sep. 30, 2016 USD ($)	Dec. 31, 2018 USD ($) ft² a property Real_Estate_Investment Apartment_unit		Dec. 31, 2017 USD ($)		Dec. 31, 2016 USD ($) ft² property	Dec. 31, 2011 USD ($)	Dec. 31, 2015 ft² property Storey	Dec. 31, 2014 USD ($)	Oct. 15, 2014	Apr. 03, 2014 ft² property Storey	Dec. 31, 2013 USD ($)	Jan. 25, 2013 USD ($) a Storey Bed	Jul. 10, 2012 USD ($) ft² property	Jan. 20, 2011 a
Schedule Of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method																								$ 169,100,000					$ 169,100,000		$ 194,621,000		$ 281,331,000
Rentable Square Feet | ft²																								1,414,457					1,414,457
Accounts receivable																								$ 16,394,000					$ 16,394,000		17,938,000
Area of Real Estate Property | ft²																								16,777,249					16,777,249
Gross sales price																								$ 359,300,000					$ 359,300,000
Proceeds from the sale of properties																													324,090,000		171,860,000		784,331,000
Net gain on disposition of real estate																													$ 2,932,000		31,657,000		116,983,000
Number of Properties | property																								97					97
Acquisition of properties																													$ 196,625,000		72,523,000		20,406,000
Proceeds from repayment of mortgage notes receivable																													$ 192,000		151,000		0
Number of Properties Sold | property																													10
Real estate ventures aggregate indebtedness to third parties																								$ 370,319,000					$ 370,319,000		768,912,000
Net gain on real estate venture transactions																													142,233,000		80,526,000		20,000,000
Property encumbered at the time of venture sale																								$ 323,032,000					323,032,000
Proceeds from real estate venture sales																													$ 60,346,000		145,416,000		21,022,000
Number of office properties owned | property																								2					2
Assets held for sale, net																								$ 11,599,000					$ 11,599,000		392,000
Debt, net																								365,707,000					365,707,000		766,420,000
Advance received for refinancing																													455,500,000		341,000,000		195,000,000
Cash distribution from unconsolidated real estate ventures																													6,526,000		20,781,000		13,065,000
Advanced under the construction loan																													908,000		6,638,000		$ 28,610,000
Secured Debt [Member]
Schedule Of Equity Method Investments [Line Items]
Real estate property debt																								323,032,000					323,032,000		$ 320,107,000
Fairview Properties [Member]
Schedule Of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method																								11,700,000					11,700,000
Equity method investment other than temporary impairment charge																													4,100,000
Encino Trace - Land [Member]
Schedule Of Equity Method Investments [Line Items]
Rentable Square Feet | ft²																																			320,000
Number of Properties | property																																			2
Number of floors of a building | Storey																																			4
DRA Austin Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage				50.00%
Service and other revenue																													$ 3,400,000
Area of Real Estate Property | ft²				1,570,123
Number of Properties | property				12
Net gain on real estate venture transactions				$ 103,800,000																				$ 103,800,000
Percentage of ownership interests				50.00%
Aggregate purchase price				$ 535,100,000
Unaffiliated Third Party [Member]
Schedule Of Equity Method Investments [Line Items]
Percentage of ownership interests																																					50.00%
Brandywine Realty Trust [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage																															50.00%
Equity Method Investment, Additional Distribution Rights																															10.00%
DRA Advisors LLC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage																															50.00%
Metropolitan Washington, D.C. - Office [Member]
Schedule Of Equity Method Investments [Line Items]
Number of properties contributed to a joint venture | property																										8
Impairment on assets held for sale																										$ 56,900,000
Four Points Centre [Member]
Schedule Of Equity Method Investments [Line Items]
Number of Properties | property																																						2
Number of floors of a building | Storey																																						3
Evo at Cira Disposition [Member]
Schedule Of Equity Method Investments [Line Items]
Gross sales price							$ 197,500,000
Net gain on real estate venture transactions							25,700,000
Cost of equity method investment							17,300,000
Proceeds from real estate venture sales							$ 43,000,000.0
Sale of ownership interest percentage							50.00%
Number of property units sold | property							345
Evo at Cira Disposition [Member] | Philadelphia [Member] | Pennsylvania [Member]
Schedule Of Equity Method Investments [Line Items]
Construction Loan							$ 110,900,000
TB-BDN Plymouth Apartments [Member]
Schedule Of Equity Method Investments [Line Items]
Gross sales price												$ 100,500,000
Number of Properties Sold | property												398
Net gain on real estate venture transactions												$ 14,600,000
Percentage of ownership interests												50.00%
Proceeds from real estate venture sales												$ 27,200,000
Sale of ownership interest percentage												50.00%
Allocated gross sales value of ownership interest, percentage												50.00%
Guarantee obligations cancelled												$ 3,200,000
TB-BDN Plymouth Apartments [Member] | Pennsylvania [Member]
Schedule Of Equity Method Investments [Line Items]
Construction Loan												54,000,000.0
Office Properties [Member]
Schedule Of Equity Method Investments [Line Items]
Rentable Square Feet | ft²																																	5,239,632
Area of Real Estate Property | ft²																								15,609,156					15,609,156
Gross sales price																																	$ 820,000,000
Proceeds from the sale of properties																																	769,799,000
Net gain on disposition of real estate	[1]																																$ 116,983,000
Number of Properties | property																								88					88
Number of Properties Sold | property																																	66
Real Estate Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Accounts receivable																								$ 800,000					$ 800,000		$ 900,000
Real Estate Venture [Member] | Management Fees [Member]
Schedule Of Equity Method Investments [Line Items]
Service and other revenue																													6,300,000		6,400,000		$ 6,700,000
Real Estate Venture [Member] | Leasing Commission Income [Member]
Schedule Of Equity Method Investments [Line Items]
Service and other revenue																													$ 2,500,000		4,500,000		$ 3,800,000
Unconsolidated Real Estate Ventures [Member]
Schedule Of Equity Method Investments [Line Items]
Number of unconsolidated investments in Real Estate Ventures | Real_Estate_Investment																								10					10
Investment in Real Estate Ventures, equity method																								$ 169,100,000					$ 169,100,000
Real estate ventures aggregate indebtedness to third parties																								370,319,000					370,319,000
Equity method investment other than temporary impairment charge	[2]																												4,076,000		4,844,000
Debt, net																								$ 365,707,000					$ 365,707,000
Unconsolidated Real Estate Ventures [Member] | TB-BDN Plymouth Apartments [Member]
Schedule Of Equity Method Investments [Line Items]
Equity in income (loss) of Real Estate Ventures												$ 12,600,000
Unconsolidated Real Estate Ventures [Member] | Minimum [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage																								15.00%					15.00%
Unconsolidated Real Estate Ventures [Member] | Maximum [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage																								70.00%					70.00%
Unconsolidated Real Estate Ventures [Member] | Six Real Estate Ventures [Member] | Office Properties [Member]
Schedule Of Equity Method Investments [Line Items]
Number of unconsolidated investments in Real Estate Ventures | Real_Estate_Investment																								6					6
Rentable Square Feet | ft²																								5,800,000					5,800,000
Unconsolidated Real Estate Ventures [Member] | Two Real Estate Ventures [Member] | Land Held For Development [Member]
Schedule Of Equity Method Investments [Line Items]
Number of unconsolidated investments in Real Estate Ventures | Real_Estate_Investment																								2					2
Acreage of land | a																								1.4					1.4
Unconsolidated Real Estate Ventures [Member] | One Real Estate Venture [Member] | Land Under Active Development [Member]
Schedule Of Equity Method Investments [Line Items]
Number of unconsolidated investments in Real Estate Ventures | Real_Estate_Investment																								1					1
Acreage of land | a																								1.3					1.3
Unconsolidated Real Estate Ventures [Member] | Two Other Real Estate Ventures [Member] | Residential Tower [Member]
Schedule Of Equity Method Investments [Line Items]
Number of unconsolidated investments in Real Estate Ventures | Real_Estate_Investment																								1					1
Number of Property units | Apartment_unit																								321					321
Herndon Innovation Center Metro Portfolio Venture, LLC [Member] | Metropolitan Washington, D.C. - Office [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage			15.00%
Number of properties contributed to a joint venture | property			8
Area of Real Estate Property | ft²			1,293,197
Gross sales price			$ 312,000,000.0
Cash proceeds received from sale of properties			265,200,000
Closing costs			2,200,000
Working capital contribution			600,000
Proceeds from the sale of properties			262,400,000
Net gain on disposition of real estate			400,000
Capital credit			47,700
Impairment on assets held for sale																										$ 56,900,000
Aggregate gross sales price of a joint venture			$ 312,000,000.0
Herndon Innovation Center Metro Portfolio Venture, LLC [Member] | Metropolitan Washington, D.C. - Office [Member] | Rockpoint [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage			85.00%
Austin Joint Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Rentable Square Feet | ft²									1,164,496											590,881	232,274
Equity method investment percentage									50.00%
Gross sales price									$ 333,250,000
Closing costs																				$ 200,000	$ 100,000
Number of Properties | property																				7	2
Equity method investment, Consideration transferred																					$ 48,200,000
Mortgage loans outstanding																					$ 34,500,000
Mortgage loans interest rate																					3.87%
Equity method investments, Cash contributions																				$ 18,900,000	$ 12,800,000
Equity Method Investments Purchase Adjustments																					900,000
Business acquisition cost of acquired entity transaction costs borne by seller																					600,000
Acquisition of properties																				128,100,000
Short-term Bank Loans and Notes Payable																				$ 88,000,000.0
Number of Properties Sold | property									8	7
Real estate ventures aggregate indebtedness to third parties									$ 150,968,000	$ 151,000,000.0
Cash proceeds after the payment of mortgage debt, closing costs and prorations									86,400,000
Net gain on real estate venture transactions									40,092,000
Deferred gain on sale									12,072,000
Proceeds from real estate venture sales									$ 172,742,000
Austin Joint Venture [Member] | Brandywine Realty Trust [Member]
Schedule Of Equity Method Investments [Line Items]
Number of properties contributed to a joint venture | property																						7
Area of Real Estate Property | ft²																						1,398,826
Aggregate gross sales price of a joint venture																						$ 330,000,000.0
Equity method investment, agreement to pay future capital expenditures																						5,200,000																	$ 800,000
Equity method investments, Cash contributions																					$ 6,400,000
Austin Joint Venture [Member] | DRA Advisors LLC [Member]
Schedule Of Equity Method Investments [Line Items]
Closing costs																						1,900,000
Equity Method Investment, Distribution, Net of Initial Funding Agreement																						266,300,000
Amount Of Participant Contribution																						49,700,000
Closing Prorations and Lender Holdbacks																						6,900,000
Proceeds from Equity Method Investment, Dividends or Distributions																						271,500,000
Non-Recourse Debt																						$ 230,600,000
Austin Joint Venture [Member] | Office Properties [Member] | Four Points Centre [Member]
Schedule Of Equity Method Investments [Line Items]
Rentable Square Feet | ft²																																						192,396
Austin, Texas [Member]
Schedule Of Equity Method Investments [Line Items]
Number of floors of a building | Storey																					3
Austin, Texas [Member] | Brandywine Realty Trust [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment, agreement to pay future capital expenditures																																				$ 4,400,000
River Place [Member]
Schedule Of Equity Method Investments [Line Items]
Proceeds from repayment of mortgage notes receivable																	$ 88,000,000.0
Brandywine - AI Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method	[3]																							$ 11,731,000					$ 11,731,000		$ 43,560,000
Rentable Square Feet | ft²		364,277									874,479
Equity method investment percentage																								50.00%	[3],[4]				50.00%	[3],[4]	50.00%		50.00%
Real estate ventures aggregate indebtedness to third parties	[3]																							$ 26,111,000					$ 26,111,000		$ 93,117,000
Impairment of properties held for use																													10,400,000
Equity method investment other than temporary impairment charge																											$ 4,800,000		$ 4,076,000	[2]	4,844,000	[2]
Number of office properties owned | property		2									5																5
Cost of equity method investment after recordation of impairment charge																											$ 44,300,000
Impairment Hold Period																											10 years		10 years
Effective interest rate	[3]																							4.65%					4.65%
Debt, net																								$ 26,020,000					$ 26,020,000		92,917,000
Brandywine - AI Venture [Member] | Residual Capitalization Rates [Member]
Schedule Of Equity Method Investments [Line Items]
Measurement input rate																								0.080			0.075		0.080
Brandywine - AI Venture [Member] | Station Square [Member]
Schedule Of Equity Method Investments [Line Items]
Rentable Square Feet | ft²																																									497,896
Equity method investment percentage																																									50.00%
Number of Properties | property																																									3
Impairment of properties held for use																												$ 10,400,000
Cost of equity method investment																																									$ 120,600,000
Equity method investment other than temporary impairment charge																												$ 5,200,000
Brandywine - AI Venture [Member] | Remaining Properties in Real Estate Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment other than temporary impairment charge																													$ 0
Brandywine - AI Venture [Member] | Station Square and 7101 Wisconsin Property Sale [Member]
Schedule Of Equity Method Investments [Line Items]
Rentable Square Feet | ft²		510,202									230,904
Equity method investment percentage		50.00%									50.00%
Gross sales price		$ 107,000,000.0									$ 105,700,000
Number of Properties Sold | property		3
Net gain on real estate venture transactions		$ 1,500,000									13,800,000
Property encumbered at the time of venture sale		66,500,000									37,400,000
Debt instrument prepayment penalty		700,000									800,000
Cash proceeds received from sale of venture		34,800,000									63,600,000
Proceeds from real estate venture sales		$ 17,400,000									$ 31,800,000
Brandywine - AI Venture [Member] | Fairview Properties [Member]
Schedule Of Equity Method Investments [Line Items]
Impairment of properties held for use																													20,800,000
Assets held for sale, net																								$ 50,400,000					50,400,000
Cost of equity method investment after recordation of impairment charge																								$ 15,800,000					$ 15,800,000
Brandywine - AI Venture [Member] | 3130 Fairview Park Drive [Member]
Schedule Of Equity Method Investments [Line Items]
Impairment Hold Period																													10 years
Brandywine - AI Venture [Member] | 3130 Fairview Park Drive [Member] | Residual Capitalization Rates [Member]
Schedule Of Equity Method Investments [Line Items]
Measurement input rate																								0.0800					0.0800
Brandywine - AI Venture [Member] | 3130 Fairview Park Drive [Member] | Discount Rates [Member]
Schedule Of Equity Method Investments [Line Items]
Measurement input rate																								0.0950					0.0950
Brandywine - AI Venture [Member] | 3141 Fairview Park Drive [Member] | Residual Capitalization Rates [Member]
Schedule Of Equity Method Investments [Line Items]
Measurement input rate																								0.0800					0.0800
Brandywine - AI Venture [Member] | 3141 Fairview Park Drive [Member] | Discount Rates [Member]
Schedule Of Equity Method Investments [Line Items]
Measurement input rate																								0.0800					0.0800
Brandywine - AI Venture [Member] | Minimum [Member] | Discount Rates [Member]
Schedule Of Equity Method Investments [Line Items]
Measurement input rate																								0.090			0.078		0.090
Brandywine - AI Venture [Member] | Maximum [Member] | Discount Rates [Member]
Schedule Of Equity Method Investments [Line Items]
Measurement input rate																								0.095			0.085		0.095
MAP Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method	[5]																							$ 11,173,000					$ 11,173,000		15,450,000
Equity method investment percentage	[4],[5]																							50.00%					50.00%
Real estate ventures aggregate indebtedness to third parties	[5]																							$ 185,000,000					$ 185,000,000		180,800,000
MAP Venture [Member] | Variable Interest Entity, Not Primary Beneficiary [Member]
Schedule Of Equity Method Investments [Line Items]
Rentable Square Feet | ft²															3,924,783
Equity method investment percentage															50.00%
Gross sales price															$ 398,100,000
Proceeds from the sale of properties															354,000,000.0
Non-Recourse Debt															$ 180,800,000
Number of Properties Sold | property															58
Percentage of fee interests in land sold															100.00%
Ground lease extended expiration date																													2115-02
Ground lease annual payments																													$ 11,900,000
Ground lease annual rent expense increase percentage through November 2025																								2.50%					2.50%
Ground Lease Terms																													Annual payments by the MAP Venture, as tenant under the Ground Leases, initially total $11.9 million and increase 2.5% annually through November 2025. Thereafter, annual rental payments increase by 2.5% or CPI at the discretion of the lessor.
Ground lease annual rent expense fixed increase percentage, end date																													2025-11
Ground lease annual rent expense increase after November 2025																								2.50%					2.50%
Ownership percentage value															$ 25,200,000
MAP Ground Lease Venture LLC [Member]
Schedule Of Equity Method Investments [Line Items]
Real estate ventures aggregate indebtedness to third parties					$ 180,800,000
Debt Instrument Maturity Date					Feb. 09, 2019
MAP Ground Lease Venture LLC [Member] | Secured Debt [Member]
Schedule Of Equity Method Investments [Line Items]
Debt Instrument Maturity Date					Aug. 01, 2023
Indebtedness to third parties, refinanced amount					$ 185,000,000.0
Debt instrument, basis spread on variable rate					2.45%
MAP Ground Lease Venture LLC [Member] | Maximum [Member] | Secured Debt [Member]
Schedule Of Equity Method Investments [Line Items]
Effective interest rate					6.00%
1919 Ventures [Member]
Schedule Of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method	[6]																							$ 19,897,000					$ 19,897,000		22,268,000
Equity method investment percentage						50.00%																		50.00%	[4],[6]				50.00%	[4],[6]												50.00%
Number of floors of a building | Storey																		29
Real estate ventures aggregate indebtedness to third parties	[6]																							$ 88,860,000					$ 88,860,000		88,860,000
Effective interest rate	[6]																							4.00%					4.00%
Payments to Acquire Land																																		$ 9,300,000
Area of developed parcels of land (in acres) owned by a Real Estate Venture | a																																										1
Remeasurement gain or loss on investments																																		$ 0
Square foot of contemporary glass tower development | ft²																		455,000
Number of luxury apartments | Apartment_unit																		321
Square feet of commercial space | ft²																		24,000
Number of Car Structured Parking Facility | NumberofCar.																		215
Development completion date																													Sep. 30, 2016
Debt, net						$ 44,400,000
Note receivable						$ 44,400,000
Loan interest rate						4.00%
Mortgage loans scheduled maturity date						Jun. 25, 2023
Repayment of construction loan						$ 88,800,000
Construction loan, principal amount						88,600,000
Accrued interest on construction loan						200,000
1919 Ventures [Member] | Other Assets [Member]
Schedule Of Equity Method Investments [Line Items]
Note receivable						44,400,000
1919 Ventures [Member] | Unaffiliated Third Party [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage																																										50.00%
1919 Ventures [Member] | LCOR/CalSTRS [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage																			50.00%
Aggregate gross sales price of a joint venture																			$ 16,400,000
Mortgage loans outstanding																															88,900,000
Equity method investments, Cash contributions																													$ 29,600,000
General Partners' Contributed Capital																			5,200,000
Net gain on sale of undepreciated real estate																			$ 8,200,000
Debt, net						$ 44,400,000
Four Tower Bridge [Member]
Schedule Of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method	[7]																							$ 0					$ 0		3,032,000
Equity method investment percentage								35.00%																65.00%	[4],[7]				65.00%	[4],[7]
Real estate ventures aggregate indebtedness to third parties	[7]																							$ 0					$ 0		9,749,000
Net gain on real estate venture transactions								$ 11,600,000
Four Tower Bridge [Member] | Conshohocken [Member] | Pennsylvania [Member]
Schedule Of Equity Method Investments [Line Items]
Area of Real Estate Property | ft²								86,021
Real estate property debt								$ 9,700,000
Seven Tower Bridge [Member]
Schedule Of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method	[7]																							$ 0					$ 0		471,000
Equity method investment percentage	[4],[7]																							20.00%					20.00%
Real estate ventures aggregate indebtedness to third parties	[7]																							$ 0					$ 0		$ 14,629,000
Sale of ownership interest percentage								20.00%
evo at Cira Centre South Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method														$ 28,300,000
Equity method investment percentage														50.00%										50.00%					50.00%		50.00%		50.00%							30.00%
Number of floors of a building | Storey																																								33
Cost of equity method investment																																								$ 4,000,000.0
Equity method investment other than temporary impairment charge	[2]																												$ 0		$ 0
Debt Instrument Maturity Date													Jul. 25, 2016
Area of developed parcels of land (in acres) owned by a Real Estate Venture | a																																								1
Debt, net																								$ 0	[8]				$ 0	[8]	110,136,000
Student housing bed capacity | Bed																																								850
Equity Method Investment, Underlying Equity in Net Assets																																							8,500,000	$ 8,500,000
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity																																							$ 4,500,000
Cash paid to acquire real estate ventures														$ 12,800,000
Equity method investment percentage of interests acquired by the Company and HSRE														30.00%
Construction Loan													$ 97,800,000
Cash distribution from unconsolidated real estate ventures													$ 6,300,000
evo at Cira Centre South Venture [Member] | Secured Debt [Member]
Schedule Of Equity Method Investments [Line Items]
Debt Instrument Maturity Date													Oct. 31, 2019
Indebtedness to third parties, refinanced amount													$ 117,000,000.0
Debt instrument, description of variable rate basis																													LIBOR + 2.25
Debt instrument optional extended maturity date													Jun. 30, 2021
Advance received for refinancing													$ 105,000,000.0
Additional capacity under the term loan may be funded if certain criterion relating to the operating performance of the student housing tower are met													$ 12,000,000.0
evo at Cira Centre South Venture [Member] | Secured Debt [Member] | LIBOR [Member]
Schedule Of Equity Method Investments [Line Items]
Debt instrument, basis spread on variable rate													2.25%
evo at Cira Centre South Venture [Member] | Campus Crest Properties, LLC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage																																								30.00%
evo at Cira Centre South Venture [Member] | HSRE, LLC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage														50.00%																										40.00%
Cash paid to acquire real estate ventures														$ 6,600,000
evo at Cira Centre South Venture [Member] | Maximum [Member] | Secured Debt [Member]
Schedule Of Equity Method Investments [Line Items]
Effective interest rate													5.25%
51 N 50 Patterson [Member]
Schedule Of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method																								$ 21,368,000					$ 21,368,000		21,212,000
Equity method investment percentage																70.00%								70.00%	[4]				70.00%	[4]
Real estate ventures aggregate indebtedness to third parties																								$ 0					$ 0		0
Payments to Acquire Land																$ 15,200,000
Area owned by real estate venture of undeveloped parcels of land | a																0.9
51 N 50 Patterson [Member] | JBG [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage																30.00%
Payments to Acquire Land																$ 6,500,000
1250 First Street Office [Member]
Schedule Of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method																								$ 17,838,000					$ 17,838,000		17,867,000
Equity method investment percentage																70.00%								70.00%	[4]				70.00%	[4]
Real estate ventures aggregate indebtedness to third parties																								$ 0					$ 0		0
Payments to Acquire Land																$ 13,200,000
Area owned by real estate venture of undeveloped parcels of land | a																0.5
1250 First Street Office [Member] | JBG [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage																30.00%
Payments to Acquire Land																$ 5,700,000
4040 Wilson Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method	[9]																							$ 37,371,000					$ 37,371,000		37,179,000
Equity method investment percentage																								50.00%	[4],[9]				50.00%	[4],[9]			50.00%
Real estate ventures aggregate indebtedness to third parties	[9]																							$ 57,288,000					$ 57,288,000		6,664,000
Area owned by real estate venture of undeveloped parcels of land | a																							1.3
Secured construction loan with total borrowing capacity																								150,000,000.0					150,000,000.0		150,000,000.0
Advanced under the construction loan																													57,300,000
Guarantees, maximum exposure amount																								41,300,000					41,300,000
4040 Wilson Venture [Member] | Mixed Use Property [Member]
Schedule Of Equity Method Investments [Line Items]
Area of Real Estate Property | ft²																							427,500
Number of Properties | property																							8
4040 Wilson Venture [Member] | Brandywine Realty Trust [Member]
Schedule Of Equity Method Investments [Line Items]
Cash paid to acquire real estate ventures																							$ 36,000,000.0
Capital contribution to venture																								36,900,000					36,900,000
4040 Wilson Venture [Member] | Ashton Park [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage																							50.00%
Capital contribution to venture																								36,900,000					36,900,000
PJP VII [Member]
Schedule Of Equity Method Investments [Line Items]
Investment in Real Estate Ventures, equity method																								$ 1,100,000					$ 1,100,000		1,156,000
Equity method investment percentage	[4]																							25.00%					25.00%
Real estate ventures aggregate indebtedness to third parties																								$ 3,777,000					$ 3,777,000		$ 4,792,000
Construction loan, principal amount																								3,800,000					3,800,000
Guarantees, maximum exposure amount																								$ 300,000					$ 300,000

[1] Gain/(Loss) on Sale is net of closing and other transaction related costs.

[2] See “Brandywine - AI Venture: Other Than Temporary Impairment” section below.

[3] See “Brandywine - AI Venture: Station Square and 7101 Wisconsin Avenue” and “Brandywine - AI Venture: Other Than Temporary Impairment” sections below for information discussing activity that occurred during 2018 and 2017 relating to this venture.

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

[5] On August 1, 2018, the MAP Venture refinanced its $180.8 million third party debt financing, secured by the buildings of MAP Venture and maturing February 9, 2019, with $185.0 million third party debt financing, also secured by the buildings, bearing interest at LIBOR + 2.45% capped at a total maximum interest of 6.00% and maturing on August 1, 2023.

[6] The basis difference associated with this venture is 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).

[7] On January 5, 2018, the Company exchanged its 20% interest in Seven Tower Bridge to acquire the remaining 35% interest in Four Tower Bridge. For further information regarding the accounting of the transaction, see “Four Tower Bridge Acquisition” section below.

[8] On January 10, 2018, HSRE-BDN I, LLC (evo at Cira Centre South) sold the 345-unit student housing tower, its sole operating asset. See “evo at Cira Disposition” section below.

[9] During the fourth quarter of 2017, 4040 Wilson obtained a secured construction loan with a total borrowing capacity of $150.0 million.