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Properties, Net (Narrative) (Details) ft² in Thousands |  | 3 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Oct. 30, 2020 USD ($) property | Sep. 30, 2020 USD ($) | Dec. 31, 2018 USD ($) ft² property | Dec. 31, 2019 USD ($) ft² property | Dec. 31, 2018 USD ($) ft² property | Dec. 31, 2020 ft² property | Dec. 22, 2020 | Dec. 05, 2019 property | Jun. 20, 2019 property | Oct. 27, 2017 USD ($) ft²
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership percentage sold | 90.00% |  |  | 90.00% |  |  |  |  |  |
Number of properties sold | 2 |  |  | 9 |  |  |  |  |  |
B RE COPT DC JV II LLC |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership percentage | 10.00% |  |  |  |  | 10.00% | 10.00% |  |  |
Number of real estate properties |  |  |  |  |  | 8 |  |  |  |
BREIT COPT DC JV LLC |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  | 10.00% |  | 10.00% |  |  |  |
Number of real estate properties |  |  |  |  |  | 9 |  |  |  |
Baltimore, Maryland |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |
Impairment charges | $ |  | $ 1,500,000 |  |  |  |  |  |  |  |
Expansion of Fully-Operational Properties Placed in Service |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |
Area of real estate property (in sqft or acres) | ft² |  |  |  |  |  | 42 |  |  |  |
Number of real estate properties |  |  |  |  |  | 3 |  |  |  |
Properties Under Development, Partially-Operational |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |
Number of real estate properties |  |  |  |  |  | 3 |  |  |  |
Newly-developed properties placed in service |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |
Area of real estate property (in sqft or acres) | ft² |  |  | 666 | 1,100 | 666 | 1,800 |  |  |  |
Number of real estate properties |  |  | 6 | 9 | 6 | 11 |  |  |  |
Redeveloped Property Placed in Service |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |
Area of real estate property (in sqft or acres) | ft² |  |  |  |  |  | 21 |  |  |  |
Properties under development |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |
Area of real estate property (in sqft or acres) | ft² |  |  |  |  |  | 1,500 |  |  |  |
Number of real estate properties |  |  |  |  |  | 11 |  |  |  |
Newly redeveloped properties placed In service |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |
Area of real estate property (in sqft or acres) | ft² |  |  | 22 | 85 | 22 |  |  |  |  |
Number of real estate properties |  |  | 1 | 1 | 1 |  |  |  |  |
Fort Meade/BW Corridor sub-segment |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |
Impairment charges | $ |  |  |  |  | $ 2,400,000 |  |  |  |  |
Data Center Shells in Northern Virginia | Disposed of by sale |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership percentage sold | 90.00% |  |  | 90.00% |  |  |  |  |  |
Number of properties sold | 2 |  |  | 9 |  |  |  | 2 | 7 |
Transaction value | $ | $ 89,700,000 |  |  | $ 345,100,000 |  |  |  |  |  |
Proceeds from sale | $ | 80,700,000 |  |  | 310,600,000 |  |  |  |  |  |
Gain on disposal | $ | $ 30,000,000.0 |  |  | $ 105,200,000 |  |  |  |  |  |
Data Center Shells in Chester, Virginia | Disposed of by sale |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |
Transaction value | $ |  |  |  |  |  |  |  |  |  | $ 44,000,000.0
Gain on disposal | $ |  |  | $ 1,500,000 |  |  |  |  |  |  |
Area of real estate property (in sqft or acres) | ft² |  |  |  |  |  |  |  |  |  | 193
Maximum exposure | $ |  |  |  |  |  |  |  |  |  | $ 20,000,000

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