Quarterly report pursuant to Section 13 or 15(d)

Equity (Details)

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Equity (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Class of Stock [Line Items]        
Dividends declared per share (in usd per share) $ 0.2750 $ 0.275 $ 0.825 $ 0.825
Conversion of common units to common shares (in units/shares)     1,895,627 337,000
Common Shares | Forward Equity Sale Agreement        
Class of Stock [Line Items]        
Number of shares issued     4,500,000  
Proceeds from common shares issued under forward equity sale agreements     $ 132.5  
Remaining capacity under forward equity sale agreements 3,000,000   3,000,000  
Common Shares | Common Stock Issued to Public Under At-the-Market Program        
Class of Stock [Line Items]        
Number of shares issued     991,664  
Weighted average price per share (in dollars per share) $ 30.46   $ 30.46  
Proceeds from Issuance of Common Stock, Net of Issuance Costs     $ 29.8  
Payments of commissions to sales agents     0.5  
At-the-market stock, offering program established, remaining capacity $ 39.8   $ 39.8  
Corporate Office Properties, L.P.        
Class of Stock [Line Items]        
Dividends declared per share (in usd per share) $ 0.275 $ 0.275 $ 0.825 $ 0.825