Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies Summary of Significant Accounting Policies 4 (Details)

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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