Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies (Details)

v3.10.0.1
Summary of Significant Accounting Policies (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Sep. 30, 2018
Jan. 01, 2018
Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Minimum rental payments due   $ 88,326    
Ground Leases        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Minimum rental payments due   $ 87,500    
Accounting Standards Update 2017-05        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Effect of change on net income $ 18,000      
Accumulated Other Comprehensive Income (Loss)        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Cumulative effect of accounting change for adoption of hedge accounting guidance       $ 276
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2017-12        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Cumulative effect of accounting change for adoption of hedge accounting guidance     $ 276  
Accumulated Other Comprehensive Income (Loss) | Corporate Office Properties, L.P.        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Cumulative effect of accounting change for adoption of hedge accounting guidance       276
Accumulated Other Comprehensive Income (Loss) | Corporate Office Properties, L.P. | Accounting Standards Update 2017-12        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Cumulative effect of accounting change for adoption of hedge accounting guidance     276  
Cumulative Distributions in Excess of Net Income        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Cumulative effect of accounting change for adoption of hedge accounting guidance       (276)
Cumulative Distributions in Excess of Net Income | Accounting Standards Update 2017-12        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Cumulative effect of accounting change for adoption of hedge accounting guidance     (276)  
Common Shares | Corporate Office Properties, L.P.        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Cumulative effect of accounting change for adoption of hedge accounting guidance       $ (276)
Common Shares | Corporate Office Properties, L.P. | Accounting Standards Update 2017-12        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Cumulative effect of accounting change for adoption of hedge accounting guidance     $ (276)