Information by Business Segment - Reconciliation of Segment Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Reconciliation of segment assets to total assets
Total COPT consolidated assets	$ 4,120,189	$ 3,854,453	$ 3,855,369
Operating properties lease liabilities included in segment assets	26,382	$ 17,317
Segment assets
Reconciliation of segment assets to total assets
Total COPT consolidated assets	3,256,968		3,024,072
Operating properties lease liabilities included in segment assets	26,054		16,645
Other assets
Reconciliation of segment assets to total assets
Total COPT consolidated assets	642,182		597,301
Non-operating property assets
Reconciliation of segment assets to total assets
Total COPT consolidated assets	$ 194,985		$ 217,351