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Properties, Net (Details 5) ft² in Thousands, $ in Thousands |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jul. 21, 2016 USD ($) Property | Dec. 31, 2018 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) ft² buillding | Dec. 31, 2016 USD ($) ft² buillding | Oct. 27, 2017 USD ($) ft²
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Increase in property and redeemable noncontrolling interests in connection with property contributed into a joint venture |  |  | $ 0 | $ 0 | $ 22,600 |
Single-tenant data centers [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Transaction Value |  |  |  |  | 44,300 |
Ownership percentage sold (percent) | 50.00% |  |  |  |  |
Increase in property and redeemable noncontrolling interests in connection with property contributed into a joint venture | $ 147,600 |  |  |  |  |
Initial amount borrowed |  |  |  |  | 60,000 |
Ownership percentage | 50.00% |  |  |  |  |
Held-for-sale [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Proceeds from sale of land held-for-investment |  |  |  | 14,300 | 21,800 |
Gains on sale of other land |  |  |  | 4,200 | 7,200 |
Operating properties [Member] | Single-tenant data centers [Member] | Data Center Shells [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on Sale |  |  |  |  | 17,900 |
Number of properties contributed | Property | 6 |  |  |  |  |
Operating properties [Member] | Held-for-sale [Member] | Defense/Information Technology Sector [Member] | Data Center Shells [Member] | 11751 Meadowville Lane [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Undiscounted indemnification amount |  |  |  |  |  | $ 20,000
Gain on Sale |  | $ 1,500 |  |  |  |
Total Rentable Square Feet | ft² |  |  |  |  |  | 193
Transaction Value |  |  |  |  |  | $ 44,000
Operating properties [Member] | Disposed of by sale [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on Sale |  |  |  | $ 5,636 | $ 15,871 |
Number of Buildings | buillding |  |  |  | 12 | 21 |
Total Rentable Square Feet | ft² |  |  |  | 799 | 1,577 |
Transaction Value |  |  |  | $ 125,829 | $ 248,738 |
Operating properties [Member] | Disposed of by sale [Member] | Regional Office [Member] | Arborcrest Corporate Campus [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on Sale |  |  |  |  | $ 4,742 |
Number of Buildings | buillding |  |  |  |  | 4 |
Total Rentable Square Feet | ft² |  |  |  |  | 654 |
Transaction Value |  |  |  |  | $ 142,800 |
Operating properties [Member] | Disposed of by sale [Member] | Regional Office [Member] | 8003 Corporate Drive [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on Sale |  |  |  |  | $ 0 |
Number of Buildings | buillding |  |  |  |  | 1 |
Total Rentable Square Feet | ft² |  |  |  |  | 18 |
Transaction Value |  |  |  |  | $ 2,400 |
Operating properties [Member] | Disposed of by sale [Member] | Regional Office [Member] | 8007, 8013, 8015, 8019 and 8023-8027 Corporate Drive [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on Sale |  |  |  |  | $ 1,906 |
Number of Buildings | buillding |  |  |  |  | 5 |
Total Rentable Square Feet | ft² |  |  |  |  | 130 |
Transaction Value |  |  |  |  | $ 14,513 |
Operating properties [Member] | Disposed of by sale [Member] | Regional Office [Member] | 4940 Campbell Drive [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on Sale |  |  |  |  | $ 0 |
Number of Buildings | buillding |  |  |  |  | 1 |
Total Rentable Square Feet | ft² |  |  |  |  | 50 |
Transaction Value |  |  |  |  | $ 5,200 |
Operating properties [Member] | Disposed of by sale [Member] | Fort Meade/BW Corridor [Member] | 1341 and 1343 Ashton Road [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on Sale |  |  |  |  | $ 848 |
Number of Buildings | buillding |  |  |  |  | 2 |
Total Rentable Square Feet | ft² |  |  |  |  | 25 |
Transaction Value |  |  |  |  | $ 2,900 |
Operating properties [Member] | Disposed of by sale [Member] | Fort Meade/BW Corridor [Member] | 1302, 1304 & 1306 Concourse Drive [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on Sale |  |  |  |  | $ 8,375 |
Number of Buildings | buillding |  |  |  |  | 3 |
Total Rentable Square Feet | ft² |  |  |  |  | 299 |
Transaction Value |  |  |  |  | $ 48,100 |
Operating properties [Member] | Disposed of by sale [Member] | Fort Meade/BW Corridor [Member] | 1331 Ashton Road [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on Sale |  |  |  |  | $ 0 |
Number of Buildings | buillding |  |  |  |  | 1 |
Total Rentable Square Feet | ft² |  |  |  |  | 29 |
Transaction Value |  |  |  |  | $ 2,625 |
Operating properties [Member] | Disposed of by sale [Member] | Fort Meade/BW Corridor [Member] | 900 Elkridge Landing Road [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on Sale |  |  |  |  | $ 0 |
Number of Buildings | buillding |  |  |  |  | 1 |
Total Rentable Square Feet | ft² |  |  |  |  | 101 |
Transaction Value |  |  |  |  | $ 7,800 |
Operating properties [Member] | Disposed of by sale [Member] | Defense/Information Technology Sector [Member] | Northern Virginia Defense/IT [Member] | 3120 Fairview Park Drive [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on Sale |  |  |  | $ 0 |  |
Number of Buildings | buillding |  |  |  | 1 |  |
Total Rentable Square Feet | ft² |  |  |  | 190 |  |
Transaction Value |  |  |  | $ 39,000 |  |
Operating properties [Member] | Disposed of by sale [Member] | Defense/Information Technology Sector [Member] | Northern Virginia Defense/IT [Member] | 2900 Towerview Road [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on Sale |  |  |  |  | $ 0 |
Number of Buildings | buillding |  |  |  |  | 1 |
Total Rentable Square Feet | ft² |  |  |  |  | 151 |
Transaction Value |  |  |  |  | $ 12,100 |
Operating properties [Member] | Disposed of by sale [Member] | Defense/Information Technology Sector [Member] | Fort Meade/BW Corridor [Member] | 1334 Ashton Road [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on Sale |  |  |  | $ 0 |  |
Number of Buildings | buillding |  |  |  | 1 |  |
Total Rentable Square Feet | ft² |  |  |  | 37 |  |
Transaction Value |  |  |  | $ 2,300 |  |
Operating properties [Member] | Disposed of by sale [Member] | Defense/Information Technology Sector [Member] | Fort Meade/BW Corridor [Member] | 7320 Parkway Drive [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on Sale |  |  |  | $ 831 |  |
Number of Buildings | buillding |  |  |  | 1 |  |
Total Rentable Square Feet | ft² |  |  |  | 57 |  |
Transaction Value |  |  |  | $ 7,529 |  |
Operating properties [Member] | Disposed of by sale [Member] | Defense/Information Technology Sector [Member] | Data Center Shells [Member] | 201 Technology Drive [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on Sale |  |  |  | $ 3,625 |  |
Number of Buildings | buillding |  |  |  | 1 |  |
Total Rentable Square Feet | ft² |  |  |  | 103 |  |
Transaction Value |  |  |  | $ 29,500 |  |
Operating properties [Member] | Disposed of by sale [Member] | Other Segments [Member] | 1560 A and B Cable Ranch Road [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on Sale |  |  |  |  | $ 0 |
Number of Buildings | buillding |  |  |  |  | 2 |
Total Rentable Square Feet | ft² |  |  |  |  | 120 |
Transaction Value |  |  |  |  | $ 10,300 |
Operating properties [Member] | Disposed of by sale [Member] | White Marsh, Maryland [Member] | Regional Office and Other [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Gain on Sale |  |  |  | $ 1,180 |  |
Number of Buildings | buillding |  |  |  | 8 |  |
Total Rentable Square Feet | ft² |  |  |  | 412 |  |
Transaction Value |  |  |  | $ 47,500 |  |

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