Real Estate Investments (Acquisitions and Dispositions) (Details) (USD $)	3 Months Ended	12 Months Ended																		0 Months Ended			12 Months Ended		0 Months Ended						0 Months Ended		25 Months Ended
	Mar. 31, 2011	Dec. 31, 2012 A	Dec. 31, 2011	Dec. 31, 2010	Feb. 25, 2013 sqft	Dec. 31, 2012 TB-BDN Plymouth Apartments [Member]	Sep. 05, 2012 TB-BDN Plymouth Apartments [Member] A	Dec. 31, 2012 7400 West at Lantana [Member] sqft	Nov. 20, 2012 Wood Oak Drive Land [Member] sqft	Dec. 11, 2012 1900 Market [Member] sqft	Jan. 06, 2012 Plymouth Meeting, Pennsylvania [Member] sqft	Jun. 27, 2011 Three Greentree Center [Member] sqft	Oct. 28, 2011 Five Greentree Centre [Member]	Oct. 28, 2011 Lake Center Two [Member]	Jul. 18, 2012 Oaklands Corporate Center [Member]	Jun. 22, 2012 Pacific Ridge Corporate Center [Member] sqft	Mar. 22, 2012 South Lake at Dulles Corner [Member] sqft	Jan. 17, 2012 304 Harper Drive [Member] sqft	Nov. 22, 2011 Four Four Two Creamery Way [Member] sqft	Oct. 28, 2011 Five Greentree Center and Lake Center Two [Member] Properties sqft	Jul. 10, 2012 Brandywine - Al Venture [Member] Properties sqft	Dec. 20, 2011 Brandywine - Al Venture [Member] Years Properties sqft	Dec. 31, 2011 Brandywine - Al Venture [Member]	Dec. 31, 2012 Brandywine - Al Venture [Member] Rate	Aug. 02, 2011 3020 Market Street [Member]	Dec. 31, 2011 3020 Market Street [Member] sqft	Aug. 12, 2011 3020 Market Street [Member]	Dec. 31, 2011 Overlook I And Ii [Member]	Mar. 28, 2011 Overlook I And Ii [Member] sqft	Jan. 20, 2011 Parcel Of Land In Pa [Member] A	Dec. 20, 2011 Mortgages [Member] Brandywine - Al Venture [Member]	Oct. 14, 2011 Dallas, Texas [Member] A	Feb. 20, 2013 Partnership Interest [Member] Parcel Of Land In Pa [Member]
Significant Acquisitions and Disposals [Line Items]
Mortgage Loans on Real Estate, Weighted Average Debt Securities, Years																						7.4
Mortgage Loans on Real Estate																															$ 90,000,000
Weighted average interest rate of debt																															4.40%
Payments to Acquire Real Estate		77,555,000	40,674,000	50,681,000																		156,000,000
Proceeds from Real Estate and Real Estate Joint Ventures																					120,600,000	120,100,000
Transaction and Joint Venture Costs																						2,900,000
Gains (Losses) on Sales of Investment Real Estate		0	2,791,000	(22,000)																			100,000
Equity Method Investment, Other than Temporary Impairment																							200,000
Equity Method Investments		193,555,000	115,807,000																				24,700,000	49,169,000
Company's Ownership Percentage in the Real Estate Venture							50.00%															50.00%		50.00%
Secured Debt		442,974,000	511,061,000																				22,000,000
Proceeds from Financing Obligation																							7,500,000
Balance of the Financing Obligation		55,826,000	45,528,000																				7,500,000
Date which receivable obtained from third party in connection wtih acquisition																									Aug. 02, 2011
Lease percentage of acquired property								100.00%		76.30%											93.20%					57.40%		100.00%
Area of land acquired																														1
Business Acquisition Purchase Price Allocation Intangible Assets								8,400,000		13,500,000																	12,000,000
Business Acquisition Purchase Price Allocated To Market Lease Liabilities								1,900,000		3,800,000																	14,400,000
Business Acquisition, Purchase Price Allocation, Other Assets																											200,000
Business Acquisition Purchase Price Allocation Other Liabilities Assumed																											400,000
Other Ownership Interests, Contributed Capital							15,500,000
Number of Units in Real Estate Property							398
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures						15,200,000
Permanent Basis Adjustment Between Carrying Value And Fair Value						300,000
Net rentable space of property acquired								136,075	10	456,922	154,392										497,896	587,317				192,707			126,496
Amount Contributed Net of Closing Costs and Customary Prorations																					26,100,000	32,200,000
Acquired real estate, cost of property											9,100,000																18,800,000		12,600,000	9,300,000
Acquisition related costs								100,000		100,000	100,000																300,000			500,000
Business Acquisition Purchase Price Allocation Building								20,900,000		17,300,000																	21,400,000
Business Acquisition, Purchase Price Allocation, Land								4,700,000		7,800,000
Number of flex/office properties sold															11
Net rentable square feet of properties sold					800,556							69,300			466,719	121,381	268,240	32,978	104,500	206,243
Sales price of properties sold					121,000,000			32,100,000	2,100,000	34,800,000		5,900,000			52,700,000	29,000,000	91,100,000	3,000,000	7,400,000	22,800,000												600,000
Lease percentage of sold property												13.90%	92.20%	47.80%	81.60%	83.70%	100.00%	90.10%	100.00%
Area Owned By Real Estate Venture Of Undeveloped Parcels Of Land		24					20																									6
Number Of Office Properties Sold																				2
Business Acquisitions Equity Interest Issued Or Issuable Description																																	50% general partner interest in the venture
Consideration Received In Anticipation Of Real Estate Venture Formation																														4,900,000
Realized Gains Losses On Sales Of Investment Real Estate	$ 2,800,000
Remaining Ownership Interest In Real Estate Properties	11.00%
Number of Real Estate Properties	3																				3	3