Earnings Per Share ("EPS") and Earnings Per Unit ("EPU") (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income attributable to COPT/COPLP	$ 18,697	$ 20,579	$ 55,281	$ 59,124
Preferred share/unit dividends/distributions	0	0	0	(6,219)
Issuance costs associated with redeemed preferred shares/units	0	0	0	(6,847)
Income attributable to share-based compensation awards	(114)	(95)	(348)	(337)
Numerator for basic and diluted EPS on net income attributable to COPT/COPLP common share/unit holders	$ 18,583	$ 20,484	$ 54,933	$ 45,721
Denominator (all weighted averages):
Denominator for basic EPS/EPU (common shares/units)	104,379	99,112	102,401	98,855
Dilutive effect of share-based compensation awards	231	146	165	154
Dilutive effect of forward equity sale agreements	178	0	60	0
Denominator for diluted EPS/EPU (common shares/units)	104,788	99,258	102,626	99,009
Basic EPS:
Net income attributable to COPT/COPLP common shareholders - basic (in dollars per share/unit)	$ 0.18	$ 0.21	$ 0.54	$ 0.46
Diluted EPS:
Net income attributable to COPT/COPLP common shareholders - diluted (in dollars per share/unit)	$ 0.18	$ 0.21	$ 0.54	$ 0.46
Corporate Office Properties, L.P.
Numerator:
Net income attributable to COPT/COPLP	$ 19,242	$ 21,437	$ 57,308	$ 61,195
Preferred share/unit dividends/distributions	(165)	(165)	(495)	(6,714)
Issuance costs associated with redeemed preferred shares/units	0	0	0	(6,847)
Income attributable to share-based compensation awards	(114)	(95)	(348)	(337)
Numerator for basic and diluted EPS on net income attributable to COPT/COPLP common share/unit holders	$ 18,963	$ 21,177	$ 56,465	$ 47,297
Denominator (all weighted averages):
Denominator for basic EPS/EPU (common shares/units)	106,514	102,462	105,248	102,255
Dilutive effect of share-based compensation awards	231	146	165	154
Dilutive effect of forward equity sale agreements	178	0	60	0
Denominator for diluted EPS/EPU (common shares/units)	106,923	102,608	105,473	102,409
Basic EPS:
Net income attributable to COPT/COPLP common shareholders - basic (in dollars per share/unit)	$ 0.18	$ 0.21	$ 0.54	$ 0.46
Diluted EPS:
Net income attributable to COPT/COPLP common shareholders - diluted (in dollars per share/unit)	$ 0.18	$ 0.21	$ 0.54	$ 0.46