COPT Defense Properties Quarterly report pursuant to Section 13 or 15(d)

Properties, Net (Details 5) ft² in Thousands	9 Months Ended
Properties, Net (Details 5) ft² in Thousands	Sep. 30, 2017 ft² Property
Office properties under, or contractually committed for, construction or approved for redevelopment
Construction, Redevelopment and Dispositions
Number of real estate properties	10
Square footage of real estate properties (in square feet) \| ft²	1,087
Newly constructed properties placed in service
Construction, Redevelopment and Dispositions
Square feet of properties placed in service \| ft²	751
Number of real estate properties	5
Newly redeveloped properties placed in service
Construction, Redevelopment and Dispositions
Square feet of properties placed in service \| ft²	88
Number of real estate properties	3
Properties under construction or contractually committed for construction
Construction, Redevelopment and Dispositions
Number of real estate properties	8
Square footage of real estate properties (in square feet) \| ft²	1,051
Properties completed Held-for-future lease \| Office properties under, or contractually committed for, construction or approved for redevelopment
Construction, Redevelopment and Dispositions
Number of real estate properties	2
Properties under or approved for redevelopment
Construction, Redevelopment and Dispositions
Number of real estate properties	2
Square footage of real estate properties (in square feet) \| ft²	36