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Concentration of Rental Revenue (Details)
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12 Months Ended | |||||||
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Dec. 31, 2012
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Dec. 31, 2011
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Dec. 31, 2010
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| Concentration Risk | ||||||||
| 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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Rental revenue | Tenants
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| Concentration Risk | ||||||||
| Number of tenants from which the entity has recognized the most rental revenue aggregated for revenue concentration disclosure | 5 | |||||||
| Percentage required for qualification as major customer | 5.00% | |||||||
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Rental revenue | Tenants | United States Government
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| Concentration Risk | ||||||||
| Percentage of revenue | 18.00% | 17.00% | 16.00% | |||||
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Rental revenue | Tenants | Northrop Grumman Corporation
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| Concentration Risk | ||||||||
| Percentage of revenue | 7.00% | [1] | 8.00% | [1] | 9.00% | [1] | ||
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Rental revenue | Tenants | Booz Allen Hamilton, Inc.
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| Concentration Risk | ||||||||
| Percentage of revenue | 6.00% | 6.00% | 5.00% | |||||
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Rental revenue | Tenants | Computer Sciences Corporation
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| Concentration Risk | ||||||||
| Percentage of revenue | 5.00% | |||||||
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Rental revenue | Tenants | Five largest tenants
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| Concentration Risk | ||||||||
| Percentage of revenue | 39.00% | 38.00% | 35.00% | |||||
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Construction contract revenue | United States Government
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| Concentration Risk | ||||||||
| Percentage of revenue | 90.00% | 90.00% | 90.00% | |||||
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