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Properties, Net (Details 4) $ in Thousands |  |  | 12 Months Ended |
--- | --- | --- | --- | ---
Jul. 21, 2016 USD ($) | Aug. 28, 2015 USD ($) | Dec. 31, 2017 USD ($) ft² buillding Property | Dec. 31, 2016 USD ($) ft² buillding Property | Dec. 31, 2015 USD ($) ft² buillding Property | Oct. 27, 2017 USD ($) 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 contribution by redeemable noncontrolling interests in a joint venture |  |  | $ 0 | $ 22,600 | $ 0 |
(Loss) gain on early extinguishment of debt |  |  | $ (513) | $ (1,110) | $ 85,275 |
Operating properties [Member] | 15000 and 15010 Conference Center Drive [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Debt settled through the surrender of real estate assets |  | $ 150,000 |  |  |  |
(Loss) gain on early extinguishment of debt |  | $ 84,800 |  |  |  |
Newly-constructed properties placed in service [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Number of Buildings | Property |  |  | 8 | 6 | 7 |
Total Rentable Square Feet | ft² |  |  | 1,100,000 | 639,000 | 897,000 |
Newly redeveloped properties placed In service [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Number of Buildings | Property |  |  | 3 | 3 | 2 |
Total Rentable Square Feet | ft² |  |  | 94,000 | 61,000 | 170,000 |
Properties under construction or contractually committed for construction [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Number of Buildings | Property |  |  | 9 |  |  |
Total Rentable Square Feet | ft² |  |  | 1,100,000 |  |  |
Properties under redevelopment [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Number of Buildings | Property |  |  | 1 |  |  |
Total Rentable Square Feet | ft² |  |  | 22,000 |  |  |
Office properties under, or contractually committed for, construction or approved for redevelopment [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Number of Buildings | Property |  |  | 10 |  |  |
Total Rentable Square Feet | ft² |  |  | 1,100,000 |  |  |
Office properties under, or contractually committed for, construction or approved for redevelopment [Member] | Single-tenant data centers [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Number of Buildings | Property |  |  | 3 |  |  |
Office properties under, or contractually committed for, construction or approved for redevelopment [Member] | Properties completed Held-for-future lease [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Number of Buildings | Property |  |  | 2 |  |  |
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 redeemable noncontrolling interests 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 [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 |  |
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 | $ 18,100 |
Gains on sale of other land |  |  | $ 4,200 | $ 7,200 | $ 4,000 |
Held-for-sale [Member] | Operating properties [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] |  |  |  |  |  |
Total Rentable Square Feet | ft² |  |  |  |  |  | 193,000
Transaction Value |  |  |  |  |  | $ 44,000
Undiscounted indemnification amount |  |  |  |  |  | $ 20,000
Disposed of by sale [Member] | Operating properties [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Number of Buildings | buillding |  |  | 12 | 21 | 9 |
Total Rentable Square Feet | ft² |  |  | 799,000 | 1,577,000 | 1,597,000 |
Transaction Value |  |  | $ 125,829 | $ 248,738 | $ 347,785 |
Gain on Sale |  |  | $ 5,636 | $ 15,871 | $ 64,033 |
Disposed of by sale [Member] | Operating properties [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 |  |  |  | 654,000 |  |
Transaction Value |  |  |  | $ 142,800 |  |
Gain on Sale |  |  |  | $ 4,742 |  |
Disposed of by sale [Member] | Operating properties [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 |  |  |  | 18,000 |  |
Transaction Value |  |  |  | $ 2,400 |  |
Gain on Sale |  |  |  | $ 0 |  |
Disposed of by sale [Member] | Operating properties [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 |  |  |  | 130,000 |  |
Transaction Value |  |  |  | $ 14,513 |  |
Gain on Sale |  |  |  | $ 1,906 |  |
Disposed of by sale [Member] | Operating properties [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 |  |  |  | 50,000 |  |
Transaction Value |  |  |  | $ 5,200 |  |
Gain on Sale |  |  |  | $ 0 |  |
Disposed of by sale [Member] | Operating properties [Member] | Regional Office [Member] | 1550 Westbranch 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 |  |  |  |  | 160,000 |
Transaction Value |  |  |  |  | $ 27,800 |
Gain on Sale |  |  |  |  | $ 0 |
Disposed of by sale [Member] | Operating properties [Member] | Regional Office [Member] | 13200 Woodland Park 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 |  |  |  |  | 397,000 |
Transaction Value |  |  |  |  | $ 84,000 |
Gain on Sale |  |  |  |  | $ 42,515 |
Disposed of by sale [Member] | Operating properties [Member] | Regional Office [Member] | 9900, 9910, 9920 Franklin Square 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 |  |  |  |  | 135,000 |
Transaction Value |  |  |  |  | $ 24,150 |
Gain on Sale |  |  |  |  | $ 6,468 |
Disposed of by sale [Member] | Operating properties [Member] | Regional Office [Member] | 9690 Deereco Road and 375 W Padonia 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 |  |  |  |  | 240,000 |
Transaction Value |  |  |  |  | $ 44,500 |
Gain on Sale |  |  |  |  | $ 15,050 |
Disposed of by sale [Member] | Operating properties [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 |  |  |  | 25,000 |  |
Transaction Value |  |  |  | $ 2,900 |  |
Gain on Sale |  |  |  | $ 848 |  |
Disposed of by sale [Member] | Operating properties [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 |  |  |  | 299,000 |  |
Transaction Value |  |  |  | $ 48,100 |  |
Gain on Sale |  |  |  | $ 8,375 |  |
Disposed of by sale [Member] | Operating properties [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 |  |  |  | 29,000 |  |
Transaction Value |  |  |  | $ 2,625 |  |
Gain on Sale |  |  |  | $ 0 |  |
Disposed of by sale [Member] | Operating properties [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 |  |  |  | 101,000 |  |
Transaction Value |  |  |  | $ 7,800 |  |
Gain on Sale |  |  |  | $ 0 |  |
Disposed of by sale [Member] | Operating properties [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] |  |  |  |  |  |
Number of Buildings | buillding |  |  | 1 |  |  |
Total Rentable Square Feet | ft² |  |  | 37,000 |  |  |
Transaction Value |  |  | $ 2,300 |  |  |
Gain on Sale |  |  | $ 0 |  |  |
Disposed of by sale [Member] | Operating properties [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] |  |  |  |  |  |
Number of Buildings | buillding |  |  | 1 |  |  |
Total Rentable Square Feet | ft² |  |  | 57,000 |  |  |
Transaction Value |  |  | $ 7,529 |  |  |
Gain on Sale |  |  | $ 831 |  |  |
Disposed of by sale [Member] | Operating properties [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] |  |  |  |  |  |
Number of Buildings | buillding |  |  | 1 |  |  |
Total Rentable Square Feet | ft² |  |  | 103,000 |  |  |
Transaction Value |  |  | $ 29,500 |  |  |
Gain on Sale |  |  | $ 3,625 |  |  |
Disposed of by sale [Member] | Operating properties [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] |  |  |  |  |  |
Number of Buildings | buillding |  |  | 1 |  |  |
Total Rentable Square Feet | ft² |  |  | 190,000 |  |  |
Transaction Value |  |  | $ 39,000 |  |  |
Gain on Sale |  |  | $ 0 |  |  |
Disposed of by sale [Member] | Operating properties [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 |  |  |  | 151,000 |  |
Transaction Value |  |  |  | $ 12,100 |  |
Gain on Sale |  |  |  | $ 0 |  |
Disposed of by sale [Member] | Operating properties [Member] | Defense/Information Technology Sector [Member] | Northern Virginia Defense/IT [Member] | 15000 and 15010 Conference Center Drive [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 |  |  |  |  | 665,000 |
Transaction Value |  |  |  |  | $ 167,335 |
Gain on Sale |  |  |  |  | $ 0 |
Disposed of by sale [Member] | Operating properties [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 |  |  |  | 120,000 |  |
Transaction Value |  |  |  | $ 10,300 |  |
Gain on Sale |  |  |  | $ 0 |  |
White Marsh, Maryland [Member] | Disposed of by sale [Member] | Operating properties [Member] | Regional Office and Other [Member] |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Number of Buildings | buillding |  |  | 8 |  |  |
Total Rentable Square Feet | ft² |  |  | 412,000 |  |  |
Transaction Value |  |  | $ 47,500 |  |  |
Gain on Sale |  |  | $ 1,180 |  |  |

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