Investment in Unconsolidated Ventures (Narrative) (Details) (USD $)	6 Months Ended
	Jun. 30, 2014 acre Room Buildings sqft Real_Estate_Ventures	Jun. 30, 2013 Real_Estate_Ventures	Dec. 31, 2013	Jun. 30, 2014 Austin Joint Venture [Member]	Jun. 30, 2014 TB-BDN Plymouth Apartments [Member]	Jun. 30, 2014 Unconsolidated Real Estate Ventures [Member] sqft	Jun. 30, 2013 Unconsolidated Real Estate Ventures [Member] sqft	Jun. 30, 2014 The Grove at Cira Centre South [Member]	Jun. 30, 2014 PJP VII [Member]	Apr. 03, 2014 Four Points Centre [Member] Storey Properties sqft
Schedule of Equity Method Investments [Line Items]
Number of floors of a building										3
Company's share of equity (Company's Basis)	$ 186,042,000		$ 180,512,000	$ (1,200,000)
Number of unconsolidated investments in Real Estate Ventures	17	17
Net rentable square feet						5,900,000	6,200,000
Number of Real Estate Properties										2
Net Rentable Space										192,396
Number of real estate ventures that owns buildings	11
Number of office buildings in Real Estate Ventures	56
Aggregate Approximate Area Of Office Buildings	5,900,000
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	3,095,000	12,568,000
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%
Guarantor Obligations, Current Carrying Value					$ 3,200,000			$ 24,700,000	$ 500,000