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

Fair Value Measurements (Details 4) - Fair value measurement on a nonrecurring basis [Member] - Fair Value, Inputs, Level 3 [Member] $ in Thousands	Dec. 31, 2017 USD ($)
Discounted cash flow [Member]
Assets and liabilities measured at fair value on a recurring basis
Fair Values on Measurement Date	$ 3,850
Discounted cash flow [Member] \| Discount Rate [Member] \| Minimum [Member]
Assets and liabilities measured at fair value on a recurring basis
Measurement inputs	0.14
Discounted cash flow [Member] \| Discount Rate [Member] \| Maximum [Member]
Assets and liabilities measured at fair value on a recurring basis
Measurement inputs	0.16
Discounted cash flow [Member] \| Discount Rate [Member] \| Weighted Average [Member]
Assets and liabilities measured at fair value on a recurring basis
Measurement inputs	0.14
Discounted cash flow [Member] \| Terminal Capitalization Rate [Member] \| Weighted Average [Member]
Assets and liabilities measured at fair value on a recurring basis
Measurement inputs	0.12
Comparable sales analysis [Member]
Assets and liabilities measured at fair value on a recurring basis
Fair Values on Measurement Date	$ 1,755