Investment in Unconsolidated Ventures (Narrative) (Details) $ in Thousands					6 Months Ended
	Jun. 22, 2015 USD ($) ft²	May. 29, 2015 USD ($) a	Feb. 27, 2015 USD ($)	Jan. 30, 2015 USD ($)	Jun. 30, 2015 USD ($) a ft² Room property real_estate_venture building apartment_unit	Dec. 31, 2014 USD ($)	Jun. 30, 2014 ft² real_estate_venture	Dec. 19, 2007 ft² Properties
Schedule of Equity Method Investments [Line Items]
Number of unconsolidated investments in Real Estate Ventures | real_estate_venture					18		17
Equity method Investments including net liabilities					$ 199,800
Company's share of equity (Company's basis)					$ 201,034	$ 225,004
Number of negative investment real estate ventures | real_estate_venture					1
Number of real estate ventures that owns buildings | real_estate_venture					10
Number of office buildings in Real Estate Ventures | building					58
Aggregate approximate area of office buildings | ft²					5,600,000
Number of real estate ventures that owns undeveloped land | real_estate_venture					4
Area owned by real estate venture of undeveloped parcels of land | a					5.2
Number of real estate ventures that owns land under development | real_estate_venture					2
Area owned by real estate venture in parcels of land under development | a					21.8
Number of real estate Investments in residential towers | real_estate_venture					1
Number of apartment units in a real estate Venture | apartment_unit					345
Number of real estate ventures developed hotel property | real_estate_venture					1
Number of rooms contain in one real estate venture developed as hotel property | Room					137
Minimum percentage of unconsolidated equity method investment ownership					20.00%
Maximum percentage of unconsolidated equity method investment ownership					70.00%
Number of Properties | property					9
Rentable Square Feet | ft²					985,914
River Place [Member]
Schedule of Equity Method Investments [Line Items]
Proceeds from collection of notes receivable				$ 88,000
Coppell Associates [Member]
Schedule of Equity Method Investments [Line Items]
Company's share of equity (Company's basis)					$ (1,200)	$ (1,200)
51 N 50 Patterson [Member]
Schedule of Equity Method Investments [Line Items]
Area owned by real estate venture of undeveloped parcels of land | a		0.9
Ownership percentage		70.00%
Payments to acquire land		$ 15,200
1250 First Street Office [Member]
Schedule of Equity Method Investments [Line Items]
Area owned by real estate venture of undeveloped parcels of land | a		0.5
Ownership percentage		70.00%
Payments to acquire land		$ 13,200
GI Interchange Office LLC (DRA — N. PA) [Member]
Schedule of Equity Method Investments [Line Items]
Number of Properties | Properties								29
Rentable Square Feet | ft²								1,611,961
Non-recourse debt			$ 174,200
Lender extension term			60 days
Unconsolidated Real Estate Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Net rentable square feet | ft²					5,600,000		5,900,000
TB-BDN Plymouth Apartments [Member]
Schedule of Equity Method Investments [Line Items]
Guarantor obligations, current carrying value					$ 3,200
The Grove at Cira Centre South [Member]
Schedule of Equity Method Investments [Line Items]
Guarantor obligations, current carrying value					24,700
PJP VII [Member]
Schedule of Equity Method Investments [Line Items]
Guarantor obligations, current carrying value					$ 500
JBG [Member] | 51 N 50 Patterson [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage		30.00%
Payments to acquire land		$ 6,500
JBG [Member] | 1250 First Street Office [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage		30.00%
Payments to acquire land		$ 5,700
Brandywine Realty Trust [Member] | GI Interchange Office LLC (DRA — N. PA) [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage								20.00%
Broadmoor Austin Associates [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage	50.00%
Payments to acquire land	$ 143,800
Rentable Square Feet | ft²	1,112,236