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Nature Of Business (Details) (USD $)	12 Months Ended			0 Months Ended
Nature Of Business (Details) (USD $)	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Sep. 02, 2011 Dulles Station, Phase I [Member]	Jun. 18, 2010 Parklawn Portfolio [Member]	Dec. 21, 2010 The Ridges [Member]	May 13, 2009 Avondale [Member]	Dec. 22, 2010 Ammendale I and II and Amvax [Member]	Jul. 23, 2009 Tech 100 Industrial Park [Member]	Jul. 31, 2009 Brandywine Center [Member]	Nov. 13, 2009 Crossroads Distribution Center [Member]	Dec. 31, 2011 Taxable Reit Subsidiary [Member]
Percentage of Distribution of Ordinary Taxable Income	90.00%
Gain and Sale	$ 97,491,000	$ 21,600,000	$ 13,300,000	$ 0	$ 7,900,000	$ 4,500,000	$ 6,700,000	$ 9,200,000	$ 4,100,000	$ 1,000,000	$ 1,500,000
Income Tax Expense (Benefit)	0
Discontinued Operation, Tax Effect of Income (Loss) from Disposal of Discontinued Operation	1,138,000	0	0
Real estate impairment	14,526,000	0	0
Deferred Tax Assets	5,700,000
Deferred Tax Assets, Net												100,000
Deferred Tax Liabilities												$ 500,000
Ordinary income	60.00%	55.00%	75.00%
Return of capital	17.00%	31.00%	17.00%
Qualified dividends	5.00%	0.00%	0.00%
Unrecaptured Section 1250 gain	13.00%	11.00%	7.00%
Capital gain	5.00%	3.00%	1.00%