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	12 Months Ended
	Dec. 31, 2016 USD ($)
Discounted cash flow [Member]
Assets and liabilities measured at fair value on a recurring basis
Fair Values on Measurement Date	$ 77,203
Discounted cash flow [Member] \| Minimum [Member]
Assets and liabilities measured at fair value on a recurring basis
Discount rate (percent)	9.30%
Terminal capitalization rate (percent)	7.80%
Discounted cash flow [Member] \| Maximum [Member]
Assets and liabilities measured at fair value on a recurring basis
Discount rate (percent)	11.30%
Terminal capitalization rate (percent)	8.50%
Discounted cash flow [Member] \| Weighted Average [Member]
Assets and liabilities measured at fair value on a recurring basis
Discount rate (percent)	9.90%
Terminal capitalization rate (percent)	8.10%
Yield analyses [Member]
Assets and liabilities measured at fair value on a recurring basis
Fair Values on Measurement Date	$ 2,482
Investor yield requirement (percent)	9.00%