Investment in Unconsolidated Ventures (Narrative) (Details) $ in Thousands				9 Months Ended
	Jun. 22, 2015 USD ($) ft²	May. 29, 2015 USD ($) a	Jan. 30, 2015 USD ($)	Sep. 30, 2015 USD ($) a ft² apartment_unit real_estate_venture Room building	Dec. 31, 2014 USD ($)	Sep. 30, 2014 ft² real_estate_venture	Dec. 19, 2007 ft² property
Schedule of Equity Method Investments [Line Items]
Number of unconsolidated investments in Real Estate Ventures | real_estate_venture				17		17
Equity method Investments including net liabilities				$ 210,700
Company's share of equity (Company's basis)				211,771	$ 225,004
Other liabilities				$ 41,630	37,558
Number of negative investment real estate ventures | real_estate_venture				1
Number of real estate ventures that owns buildings | real_estate_venture				9
Number of office buildings in Real Estate Ventures | building				29
Aggregate approximate area of office buildings | ft²				4,000,000
Number of real estate ventures that owns undeveloped land | real_estate_venture				3
Area owned by real estate venture of undeveloped parcels of land | a				4.4
Number of real estate ventures that owns land under development | real_estate_venture				3
Area owned by real estate venture in parcels of land under development | a				22.5
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%
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]
Other liabilities				$ 1,100	$ 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
25 M Street [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value of assets				$ 20,500
GI Interchange Office LLC (DRA — N. PA) [Member]
Schedule of Equity Method Investments [Line Items]
Number of Properties | property							29
Rentable Square Feet | ft²							1,611,961
Unconsolidated Real Estate Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Net rentable square feet | ft²				4,000,000		5,900,000
The Grove at Cira Centre South [Member]
Schedule of Equity Method Investments [Line Items]
Guarantor obligations, current carrying value				$ 24,700
TB-BDN Plymouth Apartments [Member]
Schedule of Equity Method Investments [Line Items]
Guarantor obligations, current carrying value				3,200
1919 Venture [Member]
Schedule of Equity Method Investments [Line Items]
Guarantor obligations, current carrying value				88,900
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