Summary of Significant Accounting Policies Summary of Significant Accounting Policies 4 (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Noncontrolling Interest [Line Items]
Preferred units in the Operating Partnership	$ 8,800	$ 8,800
Redeemable Noncontrolling Interest [Roll Forward]
Redeemable noncontrolling interest, beginning of period	8,908	9,000
Net loss attributable to noncontrolling interest	(2,565)	(1,407)
Increase in redeemable noncontrolling interest and decrease in shareholders’ equity in connection with adjustment to arrive at fair value of noncontrolling interest	3,955	1,315	0
Redeemable noncontrolling interest, end of period	10,298	8,908	9,000
Corporate Office Properties, L.P. | Series I Preferred Units
Noncontrolling Interest [Line Items]
Limited Partners' Capital Account, Units Issued	352,000
Preferred units in the Operating Partnership	$ 8,800
Preferred Stock, Liquidation Preference Per Share	$ 25.00
Annual Dividend Yield	7.50%
Annual Cumulative Preferred Return Increment Frequency	5 years
Common Units Conversion Basis Units Issuable	0.5