Real Estate Investments Acquisitions and Dispositions (Details) (USD $)	3 Months Ended		9 Months Ended			3 Months Ended			0 Months Ended	12 Months Ended			0 Months Ended		0 Months Ended		9 Months Ended				0 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Sep. 30, 2014 Campus Pointe [Member] sqft	Sep. 30, 2014 Valleybrooke Office Park [Member] sqft Properties	Apr. 16, 2014 West Point II Land [Member] acre	Apr. 07, 2014 Princeton Pike Corporate Center [Member]	Feb. 25, 2014 Princeton Pike Corporate Center [Member]	Feb. 25, 2013 Princeton Pike Corporate Center [Member] Properties sqft	Apr. 03, 2014 Four Points Centre [Member] Properties Storey sqft	Mar. 27, 2014 Rob Roy - Land [Member] acre	Apr. 03, 2014 Austin Joint Venture [Member] Four Points Centre [Member]	Feb. 19, 2014 Encino Trace - Land [Member] sqft Properties acre Storey	Sep. 30, 2014 Encino Trace - Building I [Member]	Sep. 30, 2014 One and Two Commerce Square [Member]	Dec. 31, 2013 One and Two Commerce Square [Member]	Dec. 19, 2013 One and Two Commerce Square [Member] Storey	Apr. 03, 2014 DRA Advisors LLC [Member] Austin Joint Venture [Member] Four Points Centre [Member]	Apr. 03, 2014 Brandywine Realty Trust [Member] Austin Joint Venture [Member] Four Points Centre [Member]
Significant Acquisitions and Disposals [Line Items]
Aggregate gross sales price of a joint venture																					$ 41,500,000
Net gain from remeasurement of investment in RE ventures	0	0	458,000	7,847,000													500,000
Business Combination, Step Acquisition, Equity Interest in Acquiree, Percentage																			25.00%
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage																			99.00%
Area Owned by Company of Undeveloped Parcels Of Land															54.1
Total potential development capacity															320,000
Number of Real Estate Properties															2
Number of floors of a building												3			4				41
Business Acquisition, Transaction Costs											2,000,000
Net Rentable Space												192,396
Accounts receivable, net																	2,100,000	1,600,000
Area of Land								5.3					16.8
Business Combination, Consideration Transferred															14,000,000
Construction in Progress, Gross															8,400,000	28,700,000
Land	675,109,000		675,109,000		680,513,000										4,600,000
Number of real estate properties sold											8
Net Rentable Space Sold						172,943	279,934				800,546
Escrow Deposit															1,000,000
Sale Consideration Of Sold Property						42,500,000	37,900,000	1,600,000			121,000,000		3,500,000
Provision for impairment on assets held for sale	1,765,000	0	1,765,000	0			1,800,000
Net gain on disposition of real estate	4,698,000	0	4,698,000	0		4,700,000
Disposal group, number of properties held for sale							5
Net gain (loss) on disposition of discontinued operations	(3,000)	(16,000)	900,000	3,029,000					900,000	5,300,000
Number of Real Estate Properties												2
Gain (Loss) on sale of undepreciated real estate	0	(129,000)	1,184,000	(129,000)									1,200,000
Non-Recourse Debt														29,000,000
Amount Of Participant Contribution																				5,900,000
Proceeds from Equity Method Investment, Dividends or Distributions			7,341,000	3,661,000																	34,400,000
Equity method investment, credit to capital contribution																					5,900,000
Net gain (loss) on real estate venture transactions	$ 0	$ 0	$ (417,000)	$ 3,683,000																	$ (200,000)