Annual report pursuant to Section 13 and 15(d)

Equity - COPT and Subsidiaries (Details)

v3.8.0.1
Equity - COPT and Subsidiaries (Details) - USD ($)
 1 Months Ended 3 Months Ended 12 Months Ended
Dec. 27, 2017
Nov. 02, 2017
Jun. 27, 2017
Jan. 21, 2017
Dec. 31, 2017
Sep. 30, 2016
Oct. 31, 2012
Dec. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Preferred Stock                      
Stock redeemed or called during period value                 $ 172,500,000 $ 26,583,000  
Common Shares                      
Common shares issued under forward equity sale agreements                 $ 49,944,000    
Number of common shares for each converted common unit (in shares)                 1    
Number of operating partnerships units converted into common shares (in units)                 339,513 87,000 160,160
Dividends declared per common share (in dollars per share)                 $ 1.1 $ 1.10 $ 1.10
Common Stock Issued to Public Under At-the-Market Program [Member]                      
Common Shares                      
At-market-stock, offering program established, aggregate value           $ 200,000,000 $ 150,000,000.0        
Shares issued to the public               3,720,000 591,042.000 3,720,000 890,241
Net proceeds from shares issued               $ 109,100,000 $ 19,700,000   $ 26,600,000
Payments of stock issuance costs               $ 900,000 300,000 $ 900,000 $ 400,000
At-market stock, offering program established, remaining capacity         $ 70,000,000       $ 70,000,000    
Issuance of stock, weighted average price per share (in dollars per share)         $ 33.84     $ 29.56 $ 33.84 $ 29.56 $ 30.29
Common Shares [Member]                      
Common Shares                      
Common shares issued under forward equity sale agreements                 $ 17,000    
Forward Equity Sale Agreement [Member]                      
Common Shares                      
Shares issued to the public 1,700,000       1,700,000            
Forward equity sale agreements number of shares available for issuance   9,200,000                  
Net proceeds from issue of shares after underwriter discounts but before offering expenses   $ 285,200,000                  
Initial Gross Offering Price Per Share   $ 31.00                  
Common shares issued under forward equity sale agreements $ 50,000,000       $ 50,000,000            
Preferred Shares [Member]                      
Preferred Stock                      
Number of preferred shares authorized         25,000,000       25,000,000    
Number of preferred shares of beneficial interest authorized (in dollars per share)         $ 0.01       $ 0.01    
Series K Preferred Shares [Member]                      
Preferred Stock                      
Annual dividend yield       5.60%              
Issuance costs associated with redeemed preferred shares               $ 17,000      
Series L [Member]                      
Preferred Stock                      
Annual dividend yield     7.375%                
Issuance costs associated with redeemed preferred shares                 $ 6,800,000