Investment in Unconsolidated Ventures (Narrative) (Details) (USD $)	3 Months Ended
	Mar. 31, 2014 sqft Room Buildings Real_Estate_Ventures acre	Mar. 31, 2013	Dec. 31, 2013	Mar. 13, 2013 Real_Estate_Ventures
Schedule of Equity Method Investments [Line Items]
Company's share of equity (Company's Basis)	$ 180,237,000		$ 180,512,000
Loss on real estate venture transactions	(135,000)	0
Number of unconsolidated investments in Real Estate Ventures	17			19
Number of real estate ventures that owns buildings	11
Number of office buildings in Real Estate Ventures	55
Aggregate Approximate Area Of Office Buildings	5,700,000
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	808,000	7,039,000
Number of real estate ventures that owns land under development	3
Area Owned By Real Estate Venture in Parcels Of Land Under Development	22.5
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%
TB-BDN Plymouth Apartments [Member]
Schedule of Equity Method Investments [Line Items]
Guarantor Obligations, Current Carrying Value	3,200,000
Unconsolidated Real Estate Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Net rentable square feet	5,700,000			6,400,000
The Grove at Cira Centre South [Member]
Schedule of Equity Method Investments [Line Items]
Guarantor Obligations, Current Carrying Value	24,700,000
PJP VII [Member]
Schedule of Equity Method Investments [Line Items]
Guarantor Obligations, Current Carrying Value	$ 500,000