Annual report pursuant to Section 13 and 15(d)

Selected Quarterly Financial Data (Unaudited) (Details)

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Selected Quarterly Financial Data (Unaudited) (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Condensed Financial Statements, Captions [Line Items]                      
Real estate rental revenue $ 77,071 [1],[2] $ 77,108 [1],[2] $ 75,590 [1],[2] $ 75,214 [1],[2] $ 75,413 [1],[2] $ 70,550 [1],[2] $ 70,321 [1],[2] $ 67,872 [1],[2] $ 304,983 $ 284,156 $ 253,127
Income (loss) from continuing operations 1,248 [1],[2] 5,323 [1],[2] 5,694 [1],[2] 4,834 [1],[2] (10,844) [1],[2],[3] 2,275 [1],[2] 3,963 [1],[2] 1,708 [1],[2] 17,099 (2,898) (609)
Effect of disposal of industrial segment on net income 0 0 0 0 399 4,388 5,978 5,719      
Net income 2,958 [1],[2],[3] 9,561 [1],[2] 6,008 [1],[2] 5,181 [1],[2] 31,098 [1],[2] 63,036 [1],[2] 6,556 [1],[2] 4,688 [1],[2] 23,708 105,378 37,559
Net income attributable to the controlling interests 2,958 [1],[2] 9,561 [1],[2] 6,008 [1],[2] 5,181 [1],[2] 30,689 [1],[2] 63,008 [1],[2] 6,522 [1],[2] 4,665 [1],[2] 23,708 104,884 37,426
Continuing operations - basic (in dollars per share) $ 0.02 [1],[2] $ 0.08 [1],[2] $ 0.08 [1],[2] $ 0.07 [1],[2] $ (0.16) [1],[2] $ 0.03 [1],[2] $ 0.08 [1],[2] $ 0.03 [1],[2] $ 0.25 $ (0.04) $ (0.01)
Continuing operations - diluted (in dollars per share) $ 0.02 [1],[2] $ 0.08 [1],[2] $ 0.08 [1],[2] $ 0.07 [1],[2] $ (0.16) [1],[2] $ 0.03 [1],[2] $ 0.08 [1],[2] $ 0.03 [1],[2] $ 0.25 $ (0.04) $ (0.01)
Net income per share, Basic $ 0.04 [1],[2] $ 0.14 [1],[2] $ 0.09 [1],[2] $ 0.08 [1],[2] $ 0.46 [1],[2] $ 0.95 [1],[2] $ 0.10 [1],[2] $ 0.07 [1],[2] $ 0.35 $ 1.58 $ 0.60
Net income per share, Diluted $ 0.04 [1],[2] $ 0.14 [1],[2] $ 0.09 [1],[2] $ 0.08 [1],[2] $ 0.46 [1],[2] $ 0.95 [1],[2] $ 0.10 [1],[2] $ 0.07 [1],[2] $ 0.35 $ 1.58 $ 0.60
Real estate impairment                 2,097 14,526 0
Dulles Station II [Member]
                     
Condensed Financial Statements, Captions [Line Items]                      
Real estate impairment         14,500            
4661 Kenmore Avenue [Member]
                     
Condensed Financial Statements, Captions [Line Items]                      
Real estate impairment $ 2,100                    
[1] With regard to per share calculations, the sum of the quarterly results may not equal full year results due to rounding.
[2] The prior quarter results have been restated to conform to the current quarter presentation. Specifically, results related to properties sold or held for sale have been reclassified into discontinued operations.
[3] The three months ended December 31, 2012 and 2011 include the impact of real estate impairments of $2.1 million and $14.5 million, respectively.