Investment in Unconsolidated Ventures (Narrative) (Details) (USD $)	9 Months Ended			0 Months Ended								0 Months Ended
	Sep. 30, 2014 Buildings acre Real_Estate_Ventures Room ApartmentUnits sqft	Sep. 30, 2013 Real_Estate_Ventures	Dec. 31, 2013	Jul. 31, 2014 Crossings at Lakeline [Member]	Jul. 31, 2014 Crossings at Lakeline [Member] sqft Storey Properties	Sep. 30, 2014 TB-BDN Plymouth Apartments [Member]	Sep. 30, 2014 Unconsolidated Real Estate Ventures [Member] sqft	Sep. 30, 2013 Unconsolidated Real Estate Ventures [Member] sqft	Sep. 30, 2014 The Grove at Cira Centre South [Member]	Sep. 30, 2014 PJP VII [Member]	Apr. 03, 2014 Four Points Centre [Member] Storey Properties sqft	Jul. 31, 2014 DRA Advisors LLC [Member] Crossings at Lakeline [Member]	Jul. 31, 2014 Brandywine Realty Trust [Member] Crossings at Lakeline [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments, Cash contributions				$ 12,800,000								$ 6,400,000	$ 6,400,000
Equity method investments, Purchase adjustments				900,000
Equity method investments, Closing costs					100,000
Equity method investments, Credit from seller				600,000
Number of floors of a building					3						3
Company's share of equity (Company's Basis)	197,539,000		180,512,000
Number of unconsolidated investments in Real Estate Ventures	17	17
Net rentable square feet							6,100,000	6,200,000
Number of Real Estate Properties					2						2
Net Rentable Area					232,274
Equity method investment, Consideration transferred				48,200,000
Net Rentable Space											192,396
Number of real estate ventures that owns buildings	11
Number of office buildings in Real Estate Ventures	58
Aggregate Approximate Area Of Office Buildings	6,100,000
Number of real estate ventures that owns undeveloped land	2
Area Owned By Real Estate Venture Of Undeveloped Parcels Of Land	3.8
Payments to Acquire Equity Method Investments	16,330,000	30,142,000
Number of real estate ventures that owns land under development	3
Area Owned By Real Estate Venture in Parcels Of Land Under Development	21.8
Number of Real Estate Investments in residential towers	1
Number of apartment units in a Real Estate Venture	345
Number of real estate ventures developed hotel property	1
Number Of Rooms Contain In One Real Estate Venture Developed As Hotel Property	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						$ 3,200,000			$ 24,700,000	$ 500,000