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, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2012
Sep. 30, 2012
Dec. 31, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Condensed Financial Statements, Captions [Line Items]                        
Real estate rental revenue $ 66,721 $ 65,828 $ 65,915 [1],[2] $ 64,560 [1],[2] $ 64,660 [1],[2] $ 64,471 [1],[2]   $ 63,073 [1],[2] $ 62,590 [1],[2] $ 263,024 $ 254,794 $ 234,733
Income (loss) from continuing operations (4,297) 1,709 1,538 [1],[2] 857 [1],[2] 384 [1],[2],[3] 2,604 [1],[2]   2,928 [1],[2] 1,852 [1],[2] (193) 7,768 (14,389)
Effect of disposal of medical office segment on net income 3,964 3,820 3,439 2,821 673 2,462   2,370 2,623      
Net income 18,908 [1],[3] 5,840 5,263 [1],[2] 7,335 [1],[2] 2,958 [1],[2],[3] 9,561 [1],[2]   6,008 [1],[2] 5,181 [1],[2] 37,346 23,708 105,378
Net income attributable to the controlling interests 18,908 [1] 5,840 5,263 [1],[2] 7,335 [1],[2] 2,958 [1],[2] 9,561 [1],[2]   6,008 [1],[2] 5,181 [1],[2] 37,346 23,708 104,884
Continuing operations - basic (in dollars per share) $ (0.06) [1] $ 0.03 [1] $ 0.02 [1],[2] $ 0.01 [1],[2] $ 0.01 [1],[2] $ 0.04 [1],[2]   $ 0.04 [1],[2] $ 0.03 [1],[2] $ 0.00 $ 0.11 $ (0.22)
Continuing operations - diluted (in dollars per share) $ (0.06) [1] $ 0.03 [1] $ 0.02 [1],[2] $ 0.01 [1],[2] $ 0.01 [1],[2] $ 0.04 [1],[2]   $ 0.04 [1],[2] $ 0.03 [1],[2] $ 0.00 $ 0.11 $ (0.22)
Net income per share, Basic $ 0.28 [1] $ 0.09 [1] $ 0.08 [1],[2] $ 0.11 [1],[2] $ 0.04 [1],[2] $ 0.14 [1],[2]   $ 0.09 [1],[2] $ 0.08 [1],[2] $ 0.55 $ 0.35 $ 1.58
Net income per share, Diluted $ 0.28 [1] $ 0.09 [1] $ 0.08 [1],[2] $ 0.11 [1],[2] $ 0.04 [1],[2] $ 0.14 [1],[2]   $ 0.09 [1],[2] $ 0.08 [1],[2] $ 0.55 $ 0.35 $ 1.58
Real estate impairment         2,097   14,526     0 0 14,526
Dulles Station II [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 includes the impact of real estate impairment of $2.1 million.