Real Estate Investments Real Estate Investments, Acquisitions and Dispositions (Details) (USD $)	3 Months Ended								12 Months Ended			0 Months Ended		0 Months Ended				0 Months Ended						12 Months Ended		0 Months Ended	12 Months Ended		0 Months Ended									0 Months Ended					12 Months Ended
	Dec. 31, 2013 acre	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013 acre	Dec. 31, 2012	Dec. 31, 2011	Dec. 19, 2013 One and Two Commerce Square [Member] Storey	Dec. 31, 2013 One and Two Commerce Square [Member] sqft	Dec. 19, 2013 Four Points Centre [Member] Storey acre sqft	Nov. 19, 2013 Cira Ground Land Parcel [Member] acre	Dec. 31, 2012 TB-BDN Plymouth Apartments [Member]	Sep. 05, 2012 TB-BDN Plymouth Apartments [Member] acre	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	Dec. 19, 2013 875 First Avenue [Member] sqft	Oct. 17, 2013 1336 Enterprise Drive [Member] sqft	Dec. 31, 2013 Dabney Land, East [Member]	Aug. 05, 2013 Dabney Land, East [Member] acre	Jun. 14, 2013 Pacific View Plaza [Member] sqft	Feb. 25, 2014 Princeton Pike Corporate Center [Member]	Feb. 25, 2013 Princeton Pike Corporate Center [Member] sqft Properties	Jul. 18, 2012 Oaklands Corporate Center [Member] sqft	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	Jun. 28, 2013 The Bluffs [Member] sqft	Jun. 28, 2013 100 Arrandale Boulevard [Member] sqft	Jun. 19, 2013 1700 Paoli Pike [Member] sqft	Jul. 10, 2012 Brandywine - Al Venture [Member] Properties sqft	Dec. 31, 2013 Brandywine - Al Venture [Member]	Apr. 25, 2013 Three Logan Square [Member] acre	Dec. 31, 2013 Secured Debt [Member]	Dec. 31, 2012 Secured Debt [Member]	Dec. 19, 2013 Secured Debt [Member] Two Commerce Square [Member] Rate	Dec. 19, 2013 Secured Debt [Member] One Commerce Square [Member] Rate	Dec. 31, 2013 Subsidiaries [Member]	Dec. 31, 2012 Subsidiaries [Member]	Dec. 31, 2011 Subsidiaries [Member]
Significant Acquisitions and Disposals [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Percentage												99.00%
Business Acquisition, Pro Forma Revenue									$ 610,979,000	$ 592,753,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate																																									3.96%	5.67%
Long-term Debt, Fair Value																																									107,900,000	130,200,000
Long-term Debt, Gross																																							669,762,000	443,942,000	112,000,000	125,100,000
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual									3,400,000
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual									200,000
Net gain from remeasurement of investments in real estate ventures									6,866,000	0	0	981,000																															6,866,000	0	0
Number of floors of a building												41		3
Area of Land														22.3	0.8										8													1.8
Significant Acquisitions and Disposals, Acquisition Costs or Sale Proceeds												331,800,000		47,300,000				32,100,000	2,100,000	34,800,000	9,100,000
Business Combination, Assumption of Existing Mortgage Debt												237,100,000
Ground Lease Buy-out, Total Cost															24,600,000																							20,800,000
Accrued Rent															12,100,000
Adjustment to Record Land Acquisition to Fair Value									7,752,000	0	0																											4,300,000					7,752,000	0
Payments to Acquire Real Estate									161,604,000	77,555,000	40,674,000																																161,604,000	77,555,000	40,674,000
Proceeds from Real Estate and Real Estate Joint Ventures																																				120,600,000
Gains (Losses) on Sales of Investment Real Estate									0	0	2,985,000																																0	0	2,985,000
Equity Method Investments	180,512,000				193,555,000				180,512,000	193,555,000	115,807,000																										46,406,000						180,512,000	193,555,000
Company's Ownership Percentage in the Real Estate Venture																	50.00%																				50.00%
Secured Debt	670,151,000				442,974,000				670,151,000	442,974,000																													670,151,000	442,974,000			670,151,000	442,974,000
Balance of the Financing Obligation	32,923,000				55,826,000				32,923,000	55,826,000																																	32,923,000	55,826,000
Lease percentage of acquired property													86.70%	99.20%
Business Acquisition Purchase Price Allocation Intangible Assets														6,500,000				8,400,000		13,500,000
Business Acquisition Purchase Price Allocated To Market Lease Liabilities														1,000,000				1,900,000		3,800,000
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													1,896,142	192,396				136,075	10	456,922	154,392															497,896
Amount Contributed Net of Closing Costs and Customary Prorations																																				26,100,000
Acquisition related costs												900,000		100,000	1,400,000			100,000		100,000	100,000																	100,000
Gain (Loss) on Sale of Properties									3,382,000	34,774,000	7,047,000											100,000	200,000			(500,000)	5,300,000		9,900,000	(2,800,000)	14,500,000	200,000	(900,000)	(400,000)	(400,000)								3,382,000	34,774,000	7,047,000
Business Acquisition Purchase Price Allocation Building														36,000,000				20,900,000		17,300,000
Business Acquisition Purchase Price Allocation, Land Portion														5,800,000				4,700,000		7,800,000
Number of flex/office properties sold																												8	11
Net rentable square feet of properties sold																						50,000	39,330			51,695		800,546	466,719	121,381	268,240	32,978	68,708	34,931	28,000
Sales price of properties sold																						3,800,000	2,600,000		500,000	10,300,000		121,000,000	52,700,000	29,000,000	91,100,000	3,000,000	18,000,000	3,500,000	2,700,000
Net gain (loss) on sale of undepreciated real estate									(137,000)	0	0													(100,000)																			(137,000)	0	0
Area Owned By Real Estate Venture Of Undeveloped Parcels Of Land	3.8								3.8								20
Number of Real Estate Properties																																				3
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax									21,805,000	(60,011,000)
Business Acquisition, Pro Forma Net Income (Loss)									25,957,000	(16,970,000)
Continuing operations (in dollars per share)									$ 0.20	$ (0.36)	$ (0.24)																																$ 0.20	$ (0.36)	$ (0.23)
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Basic									$ 0.14	$ (0.42)
Income (Loss) from Continuing Operations, Per Diluted Share									$ 0.20	$ (0.36)	$ (0.24)																																$ 0.20	$ (0.36)	$ (0.23)
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Diluted									$ 0.14	$ (0.42)
Earnings Per Share, Basic	$ 0.12	$ 0.06	$ 0.03	$ 0.01	$ (0.21)	$ 0.10	$ 0.01	$ 0.05	$ 0.23	$ (0.06)	$ (0.10)																																$ 0.23	$ (0.06)	$ (0.09)
Pro Forma, Earnings Per Share, Basic									$ 0.17	$ (0.12)
Earnings Per Share, Diluted	$ 0.12	$ 0.06	$ 0.03	$ 0.01	$ (0.21)	$ 0.10	$ 0.01	$ 0.05	$ 0.23	$ (0.06)	$ (0.10)																																$ 0.23	$ (0.06)	$ (0.09)
Pro Forma Earnings Per Share, Diluted									$ 0.17	$ (0.12)
Settlement period to finalize a business combination												60 days
Other Assets	$ 62,377,000				$ 53,325,000				$ 62,377,000	$ 53,325,000		$ 2,100,000																															$ 62,377,000	$ 53,325,000