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

Properties, Net (Details 2)

Properties, Net (Details 2) - Disposed of by sale ft² in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) ft² property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties \| property	12
Total Rentable Square Feet \| ft²	799
Transaction Value	$ 125,829
Gain on Sale	$ 5,636
3120 Fairview Park Drive
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties \| property	1
Total Rentable Square Feet \| ft²	190
Transaction Value	$ 39,000
Gain on Sale	$ 0
1334 Ashton Road
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties \| property	1
Total Rentable Square Feet \| ft²	37
Transaction Value	$ 2,300
Gain on Sale	$ 0
White Marsh, Maryland
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties \| property	8
Total Rentable Square Feet \| ft²	412
Transaction Value	$ 47,500
Gain on Sale	$ 1,180
201 Technology Drive
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties \| property	1
Total Rentable Square Feet \| ft²	103
Transaction Value	$ 29,500
Gain on Sale	$ 3,625
7320 Parkway Drive
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties \| property	1
Total Rentable Square Feet \| ft²	57
Transaction Value	$ 7,529
Gain on Sale	$ 831