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Fair Value Measurements (Details 3) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 USD ($) | Jun. 30, 2017 USD ($) | Dec. 31, 2016 USD ($) | Sep. 30, 2016 USD ($) | Jun. 30, 2016 USD ($) | Mar. 31, 2016 USD ($) Property | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Assets held for sale, net | $ 42,226 |  | $ 94,654 |  |  |  | $ 42,226 | $ 94,654 |
Impairment losses |  |  |  |  |  |  | 15,123 | 101,391 | $ 23,289
Held-for-sale [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Assets held for sale, net | 42,226 |  | 94,654 |  |  |  | 42,226 | 94,654 |
Fort Meade/BW Corridor [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Number of properties | Property |  |  |  |  |  | 2 |  |  |
Fair value measurement on a nonrecurring basis [Member] | Held-for-sale [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Assets held for sale, net |  |  | 94,700 | $ 161,500 | $ 300,600 | $ 225,900 |  | $ 94,700 | 96,800
Impairment losses |  |  |  | 11,500 |  | $ 2,400 |  |  |
Fair value measurement on a nonrecurring basis [Member] | Disposed of by sale [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Impairment losses |  |  | 300 |  |  |  |  |  |
Transaction value of operating property dispositions |  |  | 54,100 | 210,700 |  |  |  |  |
Fair value measurement on a nonrecurring basis [Member] | Operating properties, net [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Assets | 3,850 |  |  |  |  |  | 3,850 |  |
Fair value measurement on a nonrecurring basis [Member] | Operating properties, net [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Assets | 3,850 |  |  |  |  |  | 3,850 |  |
Fair value measurement on a nonrecurring basis [Member] | Projects in development or held for future development [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Assets | 1,755 |  |  |  |  |  | 1,755 |  |
Fair value measurement on a nonrecurring basis [Member] | Projects in development or held for future development [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Assets | 1,755 |  |  |  |  |  | $ 1,755 |  |
Fair value measurement on a nonrecurring basis [Member] | Northern Virginia Defense/IT and Fort Meade/Baltimore Washington Corridor [Member] | Held-for-sale [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Impairment losses |  |  |  |  | 14,100 |  |  |  |
Fair value measurement on a nonrecurring basis [Member] | Defense/Information Technology Sector [Member] | Northern Virginia Defense/IT [Member] | Held-for-sale [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Impairment losses |  |  |  | 13,300 |  |  |  |  |
Fair value measurement on a nonrecurring basis [Member] | Aberdeen, Maryland [Member] | Land in development or held for future development [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Impairment of Long-Lived Assets to be Disposed of | 4,700 |  |  |  |  |  |  |  |
Impairment losses |  |  |  |  | 4,400 |  |  |  |
Fair value measurement on a nonrecurring basis [Member] | Aberdeen, Maryland [Member] | Other Segments [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Impairment of Long-Lived Assets to be Disposed of | $ 9,000 |  |  |  |  |  |  |  |
Fair value measurement on a nonrecurring basis [Member] | Frederick, Maryland [Member] | Land in development or held for future development [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Impairment losses |  |  |  |  | 8,200 |  |  |  |
Fair value measurement on a nonrecurring basis [Member] | Greater Philadelphia [Member] | Regional Office [Member] | Held-for-sale [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Impairment losses |  |  |  |  | 6,200 |  |  |  |
Fair value measurement on a nonrecurring basis [Member] | Colorado Springs, Colorado and White Marsh, Maryland [Member] | Regional Office [Member] | Held-for-sale [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Impairment losses |  |  |  |  | 2,400 |  |  |  |
Fair value measurement on a nonrecurring basis [Member] | Colorado Springs [Member] | Held-for-sale [Member] | Land in development or held for future development [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Impairment losses |  |  |  |  |  |  |  |  | 12,800
Fair value measurement on a nonrecurring basis [Member] | White Marsh, Maryland [Member] | Held-for-sale [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Impairment of Long-Lived Assets to be Disposed of |  | $ 1,600 |  |  |  |  |  |  |
Impairment losses |  |  | $ 1,300 | 10,000 |  |  |  |  |
Fair value measurement on a nonrecurring basis [Member] | White Marsh, Maryland [Member] | Held-for-sale [Member] | Land in development or held for future development [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Impairment losses |  |  |  | $ 2,900 |  |  |  |  |
Fair value measurement on a nonrecurring basis [Member] | White Marsh, Maryland [Member] | Regional Office [Member] | Held-for-sale [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Impairment losses |  |  |  |  |  |  |  |  | 2,600
Operating properties, net [Member] | Fair value measurement on a nonrecurring basis [Member] | Aberdeen, Maryland [Member] | Land in development or held for future development [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Impairment losses |  |  |  |  |  |  |  |  | 6,600
Operating properties, net [Member] | Fair value measurement on a nonrecurring basis [Member] | Aberdeen, Maryland [Member] | Other Segments [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Impairment losses |  |  |  |  | $ 34,400 |  |  |  |
Operating properties, net [Member] | Fair value measurement on a nonrecurring basis [Member] | Northern Virginia [Member] | Regional Office [Member] |  |  |  |  |  |  |  |  |
Fair value of impaired properties and other assets |  |  |  |  |  |  |  |  |
Impairment losses |  |  |  |  |  |  |  |  | $ 1,300

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