COPT Defense Properties Annual report pursuant to Section 13 and 15(d)

Properties, Net (Narrative) (Details)

Properties, Net (Narrative) (Details) ft² in Thousands, $ in Millions				3 Months Ended	12 Months Ended
	Dec. 30, 2021 USD ($)	Jun. 02, 2021 USD ($) property	Oct. 30, 2020 USD ($) property	Sep. 30, 2020 USD ($)	Dec. 31, 2021 USD ($) ft² property	Dec. 31, 2020 USD ($) ft² property	Dec. 31, 2019 USD ($) ft² property	Dec. 22, 2020	Dec. 05, 2019 property	Jun. 20, 2019 property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership percentage sold					90.00%	90.00%	90.00%
Number of properties sold					2	2	9
BRE-COPT 3
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Nominal Ownership %		10.00%			10.00%
Number of real estate properties					2
BRE-COPT 2
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Nominal Ownership %			10.00%		10.00%	10.00%		10.00%
Number of real estate properties					8
BREIT-COPT
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Nominal Ownership %					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 \| $				$ 1.5
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					766	1,800	1,100
Number of real estate properties					8	11	9
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					1,700
Number of real estate properties					11
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					1
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
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	85
Number of real estate properties							1
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%				90.00%
Number of properties sold		2	2				9		2	7
Transaction value \| $		$ 118.8	$ 89.7				$ 345.1
Proceeds from sale \| $		$ 106.9	$ 80.7				310.6
Gain on disposal \| $					$ 40.2	$ 30.0	$ 105.2
Other Data Center Shells \| Disposed of by sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale \| $	$ 30.0
Gain on disposal \| $	$ 25.9