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Properties, net (Details 4) ft² in Thousands, $ in Thousands |  | 12 Months Ended
--- | --- | ---
Jul. 21, 2016 USD ($) | Dec. 31, 2016 USD ($) ft² buillding | Dec. 31, 2015 USD ($) ft² buillding | Dec. 31, 2014 USD ($) ft² buillding
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 contribution by a redeemable noncontrolling interest in a joint venture |  | $ 22,600 | $ 0 | $ 0
Proceeds from sale of land held-for-investment |  | 21,800 |  |
Gains on sale of other land |  | $ 7,200 |  |
Operating properties, net [Member] |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |
Number of Buildings | buillding |  | 21 | 9 | 8
Total Rentable Square Feet | ft² |  | 1,577 | 1,597 | 303
Transaction Value |  | $ 248,738 | $ 347,785 | $ 28,835
Gain on Disposition |  | $ 15,871 | $ 64,033 | $ 5,093
Operating properties, net [Member] | Regional Office [Member] | Arborcrest Corporate Campus [Member] |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |
Number of Buildings | buillding |  | 4 |  |
Total Rentable Square Feet | ft² |  | 654 |  |
Transaction Value |  | $ 142,800 |  |
Gain on Disposition |  | $ 4,742 |  |
Operating properties, net [Member] | Regional Office [Member] | 8003 Corporate Drive [Member] |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |
Number of Buildings | buillding |  | 1 |  |
Total Rentable Square Feet | ft² |  | 18 |  |
Transaction Value |  | $ 2,400 |  |
Gain on Disposition |  | $ 0 |  |
Operating properties, net [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] |  |  |  |
Number of Buildings | buillding |  | 5 |  |
Total Rentable Square Feet | ft² |  | 130 |  |
Transaction Value |  | $ 14,513 |  |
Gain on Disposition |  | $ 1,906 |  |
Operating properties, net [Member] | Regional Office [Member] | 4940 Campbell Drive [Member] |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |
Number of Buildings | buillding |  | 1 |  |
Total Rentable Square Feet | ft² |  | 50 |  |
Transaction Value |  | $ 5,200 |  |
Gain on Disposition |  | $ 0 |  |
Operating properties, net [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] |  |  |  |
Number of Buildings | buillding |  | 2 |  |
Total Rentable Square Feet | ft² |  | 25 |  |
Transaction Value |  | $ 2,900 |  |
Gain on Disposition |  | $ 848 |  |
Operating properties, net [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] |  |  |  |
Number of Buildings | buillding |  | 3 |  |
Total Rentable Square Feet | ft² |  | 299 |  |
Transaction Value |  | $ 48,100 |  |
Gain on Disposition |  | $ 8,375 |  |
Operating properties, net [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] |  |  |  |
Number of Buildings | buillding |  | 1 |  |
Total Rentable Square Feet | ft² |  | 29 |  |
Transaction Value |  | $ 2,625 |  |
Gain on Disposition |  | $ 0 |  |
Operating properties, net [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] |  |  |  |
Number of Buildings | buillding |  | 1 |  |
Total Rentable Square Feet | ft² |  | 101 |  |
Transaction Value |  | $ 7,800 |  |
Gain on Disposition |  | $ 0 |  |
Operating properties, net [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] |  |  |  |
Number of Buildings | buillding |  | 1 |  |
Total Rentable Square Feet | ft² |  | 151 |  |
Transaction Value |  | $ 12,100 |  |
Gain on Disposition |  | $ 0 |  |
Operating properties, net [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] |  |  |  |
Number of Buildings | buillding |  | 2 |  |
Total Rentable Square Feet | ft² |  | 120 |  |
Transaction Value |  | $ 10,300 |  |
Gain on Disposition |  | $ 0 |  |
Operating office properties [Member] |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |
Total Rentable Square Feet | ft² |  | 17,200 |  |
Operating office properties [Member] | 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 contribution by a redeemable noncontrolling interest in a joint venture | $ 147,600 |  |  |
Initial amount borrowed |  | 60,000 |  |
Ownership percentage | 50.00% |  |  |
Operating office properties [Member] | Single-tenant data centers [Member] | Operating properties, net [Member] | Data Center Shells [Member] |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |
Gain on Disposition |  | $ 17,900 |  |
Number of properties contributed | buillding |  | 6 |  |

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