Real Estate Investments Acquisitions and Dispositions (Details) (USD $)	3 Months Ended		6 Months Ended				0 Months Ended	12 Months Ended			0 Months Ended		0 Months Ended		6 Months Ended				0 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	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] sqft Properties	Apr. 03, 2014 Four Points Centre [Member] Properties sqft Storey	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 acre Properties Storey	Jun. 30, 2014 Encino Trace - Building I [Member]	Jun. 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
Gain from remeasurement of investment in real estate ventures	458,000	7,847,000	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	21,622,000		21,622,000		17,389,000										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	18,700,000
Land	688,775,000		688,775,000		680,513,000								4,600,000
Number of real estate properties sold									8
Net Rentable Space Sold									800,546
Escrow Deposit													1,000,000
Land Sales						1,600,000			121,000,000		3,500,000
Net gain (loss) on disposition of discontinued operations	903,000	(2,259,000)	903,000	3,045,000			900,000	5,300,000
Number of Real Estate Properties										2
Gain (Loss) on sale of undepreciated real estate	(3,000)	0	1,184,000	0							1,200,000
Non-Recourse Debt												29,000,000
Amount Of Participant Contribution																		5,900,000
Proceeds from Equity Method Investment, Dividends or Distributions			5,329,000	3,445,000															34,400,000
Equity method investment, credit to capital contribution																			5,900,000
Gain (loss) on real estate venture transactions	$ (282,000)	$ 3,683,000	$ (417,000)	$ 3,683,000															$ (200,000)