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Concentration of Rental Revenue (Details) | 12 Months Ended |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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Dec. 31, 2011 region | Dec. 31, 2011 Rental revenue Tenants tenant | Dec. 31, 2011 Rental revenue Tenants United States Government | Dec. 31, 2010 Rental revenue Tenants United States Government | Dec. 31, 2009 Rental revenue Tenants United States Government | Dec. 31, 2011 Rental revenue Tenants Northrop Grumman Corporation | Dec. 31, 2010 Rental revenue Tenants Northrop Grumman Corporation | Dec. 31, 2009 Rental revenue Tenants Northrop Grumman Corporation | Dec. 31, 2011 Rental revenue Tenants Booz Allen Hamilton, Inc. | Dec. 31, 2010 Rental revenue Tenants Booz Allen Hamilton, Inc. | Dec. 31, 2009 Rental revenue Tenants Booz Allen Hamilton, Inc. | Dec. 31, 2011 Rental revenue Tenants Five largest tenants | Dec. 31, 2010 Rental revenue Tenants Five largest tenants | Dec. 31, 2009 Rental revenue Tenants Five largest tenants | Dec. 31, 2011 Construction contract revenue United States Government | Dec. 31, 2010 Construction contract revenue United States Government | Dec. 31, 2009 Construction contract revenue United States Government
Concentration Risk |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage required for qualification as major customer |  | 5.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of revenue |  |  | 17.00% | 16.00% | 15.00% | 8.00% | 9.00% | 8.00% | 6.00% | 5.00% | 6.00% | 38.00% | 35.00% | 34.00% | 90.00% | 90.00% | 90.00%
Number of tenants from which the entity has recognized the most rental revenue aggregated for revenue concentration disclosure |  | 5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of regions with real estate operations which are not located in the Mid-Atlantic region of the United States | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Represents the number of regions with real estate operations which are not located in the Mid-Atlantic region of the United States. + References No definition available. + Details
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- Definition Represents the number of tenants from which the entity has recognized the most rental revenue aggregated for revenue concentration disclosure. + References No definition available. + Details
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- Definition Threshold percentage which the entity uses for disclosure. + References No definition available. + Details
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- Definition For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6404-108592 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13537-108611 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13531-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6351-108592 + Details
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