Real Estate Investments (Brandy-Al Venture) (Details) (USD $)	12 Months Ended						0 Months Ended	12 Months Ended			0 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Aug. 12, 2011 sqft	Mar. 31, 2011	Nov. 30, 2010	Dec. 20, 2011 Brandywine - Al Venture [Member] Years Properties	Dec. 31, 2011 Brandywine - Al Venture [Member] Properties sqft		Dec. 20, 2011 Brandywine - Al Venture [Member] Current Creek [Member]	Dec. 20, 2011 Brandywine - Al Venture [Member] Mortgages [Member]
Joint venture
Company's ownership percentage in the Real Estate Venture						25.00%	50.00%	50.00%	[1],[2]	50.00%
Total mortgage indebtedness	$ 511,061,000	$ 711,789,000						$ 22,000,000
Number of office buildings in Real Estate Venture					3		3	2
Net rentable space				192,707				587,317
Amount contributed (net of closing costs and customary propations) to the capital of the Venture							32,200,000
Debt instrument, face amount											90,000,000
Debt intstrument weighted average debt securities, years							7.4
Weighted average interest rate of debt											4.40%
Payments from Joint Venture to acquire properties	40,674,000	50,681,000	0				156,000,000
Sales of properties, net							120,100,000
Transaction and joint Venture costs							2,900,000
Sourcing additional acquisitions over investment period	3 years
Net loss on partial sale of properties	(2,791,000)	0	3,000					100,000
Basis adjustment in equity method investment								100,000
Company's share of equity (Company's basis)	115,807,000	84,372,000						24,670,000	[2]
Proceeds from financing obligation								7,500,000
Balance of the financing obligation	$ 45,528,000	$ 36,515,000						$ 7,500,000

[1] Ownership percentage represents the Company’s entitlement to residual distributions after payments of priority returns, where applicable.

[2] See Note 3 for related discussion about the Brandywine-AI Venture, LLC formation.