Earnings Per Share ("EPS") and Earnings Per Unit (“EPU”) (Tables)
   | 
12 Months Ended | 
Dec. 31, 2017  | 
| Earnings Per Share [Line Items] | 
 
 | 
| Summary of calculation of numerator and denominator in basic and diluted earning per share | 
Summaries of the numerator and denominator for purposes of basic and diluted EPS calculations are set forth below (in thousands, except per share data): 
 |  
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 | 
 | 
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   | 
   | 
For the Years Ended December 31,  | 
 
   | 
   | 
2017  | 
   | 
2016  | 
   | 
2015  | 
 
Numerator:  | 
   | 
   | 
   | 
   | 
   | 
   | 
 
Income (loss) from continuing operations  | 
   | 
$  | 
66,443  | 
 
  | 
   | 
$  | 
(25,331  | 
)  | 
   | 
$  | 
120,675  | 
 
  | 
 
Gain on sales of real estate  | 
   | 
9,890  | 
 
  | 
   | 
40,986  | 
 
  | 
   | 
68,047  | 
 
  | 
 
Preferred share dividends  | 
   | 
(6,219  | 
)  | 
   | 
(14,297  | 
)  | 
   | 
(14,210  | 
)  | 
 
Issuance costs associated with redeemed preferred shares  | 
   | 
(6,847  | 
)  | 
   | 
(17  | 
)  | 
   | 
—  | 
 
  | 
 
Income from continuing operations attributable to noncontrolling interests  | 
   | 
(6,242  | 
)  | 
   | 
(4,216  | 
)  | 
   | 
(10,575  | 
)  | 
 
Income from continuing operations attributable to share-based compensation awards  | 
   | 
(449  | 
)  | 
   | 
(419  | 
)  | 
   | 
(706  | 
)  | 
 
Numerator for basic EPS from continuing operations attributable to COPT common shareholders  | 
   | 
$  | 
56,576  | 
 
  | 
   | 
$  | 
(3,294  | 
)  | 
   | 
$  | 
163,231  | 
 
  | 
 
Dilutive effect of common units in COPLP on diluted EPS from continuing operations  | 
   | 
—  | 
 
  | 
   | 
—  | 
 
  | 
   | 
6,397  | 
 
  | 
 
Numerator for diluted EPS from continuing operations attributable to COPT common shareholders  | 
   | 
$  | 
56,576  | 
 
  | 
   | 
$  | 
(3,294  | 
)  | 
   | 
$  | 
169,628  | 
 
  | 
 
Numerator for basic EPS from continuing operations attributable to COPT common shareholders  | 
   | 
$  | 
56,576  | 
 
  | 
   | 
$  | 
(3,294  | 
)  | 
   | 
$  | 
163,231  | 
 
  | 
 
Discontinued operations  | 
   | 
—  | 
 
  | 
   | 
—  | 
 
  | 
   | 
156  | 
 
  | 
 
Discontinued operations attributable to noncontrolling interests  | 
   | 
—  | 
 
  | 
   | 
—  | 
 
  | 
   | 
(3  | 
)  | 
 
Numerator for basic EPS on net income (loss) attributable to COPT common shareholders  | 
   | 
$  | 
56,576  | 
 
  | 
   | 
$  | 
(3,294  | 
)  | 
   | 
$  | 
163,384  | 
 
  | 
 
Dilutive effect of common units in COPLP  | 
   | 
—  | 
 
  | 
   | 
—  | 
 
  | 
   | 
6,403  | 
 
  | 
 
Numerator for diluted EPS on net income (loss) attributable to COPT common shareholders  | 
   | 
$  | 
56,576  | 
 
  | 
   | 
$  | 
(3,294  | 
)  | 
   | 
$  | 
169,787  | 
 
  | 
 
Denominator (all weighted averages):  | 
   | 
   | 
   | 
   | 
   | 
   | 
 
Denominator for basic EPS (common shares)  | 
   | 
98,969  | 
 
  | 
   | 
94,502  | 
 
  | 
   | 
93,914  | 
 
  | 
 
Dilutive effect of forward equity sale agreements and share-based compensation awards  | 
   | 
186  | 
 
  | 
   | 
—  | 
 
  | 
   | 
61  | 
 
  | 
 
Dilutive effect of common units  | 
   | 
—  | 
 
  | 
   | 
—  | 
 
  | 
   | 
3,692  | 
 
  | 
 
Denominator for diluted EPS (common shares)  | 
   | 
99,155  | 
 
  | 
   | 
94,502  | 
 
  | 
   | 
97,667  | 
 
  | 
 
Basic EPS:  | 
   | 
   | 
   | 
   | 
   | 
   | 
 
Income (loss) from continuing operations attributable to COPT common shareholders  | 
   | 
$  | 
0.57  | 
 
  | 
   | 
$  | 
(0.03  | 
)  | 
   | 
$  | 
1.74  | 
 
  | 
 
Net income (loss) attributable to COPT common shareholders  | 
   | 
$  | 
0.57  | 
 
  | 
   | 
$  | 
(0.03  | 
)  | 
   | 
$  | 
1.74  | 
 
  | 
 
Diluted EPS:  | 
   | 
   | 
   | 
   | 
   | 
   | 
 
Income (loss) from continuing operations attributable to COPT common shareholders  | 
   | 
$  | 
0.57  | 
 
  | 
   | 
$  | 
(0.03  | 
)  | 
   | 
$  | 
1.74  | 
 
  | 
 
Net income (loss) attributable to COPT common shareholders  | 
   | 
$  | 
0.57  | 
 
  | 
   | 
$  | 
(0.03  | 
)  | 
   | 
$  | 
1.74  | 
 
  | 
 
  
 
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| Schedule of securities excluded from computation of diluted earnings per share | 
Our diluted EPS computations do not include the effects of the following securities since the conversions of such securities would increase diluted EPS for the respective periods (in thousands):
 |  
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 | 
 | 
 | 
 | 
 | 
 | 
 | 
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   | 
   | 
Weighted Average Shares Excluded from Denominator for the Years Ended December 31,  | 
 
   | 
   | 
2017  | 
   | 
2016  | 
   | 
2015  | 
 
Conversion of common units  | 
   | 
3,362  | 
 
  | 
   | 
3,633  | 
 
  | 
   | 
—  | 
 
  | 
 
Conversion of Series I preferred units  | 
   | 
176  | 
 
  | 
   | 
176  | 
 
  | 
   | 
176  | 
 
  | 
 
Conversion of Series K preferred shares  | 
   | 
—  | 
 
  | 
   | 
434  | 
 
  | 
   | 
434  | 
 
  | 
 
  
  
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| Corporate Office Properties, L.P. [Member] | 
 
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| Earnings Per Share [Line Items] | 
 
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| Summary of calculation of numerator and denominator in basic and diluted earning per share | 
Summaries of the numerator and denominator for purposes of basic and diluted EPU calculations are set forth below (in thousands, except per unit data):
 |  
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
   | 
   | 
For the Years Ended December 31,  | 
 
   | 
   | 
2017  | 
   | 
2016  | 
   | 
2015  | 
 
Numerator:  | 
   | 
   | 
   | 
   | 
   | 
   | 
 
Income (loss) from continuing operations  | 
   | 
$  | 
66,443  | 
 
  | 
   | 
$  | 
(25,331  | 
)  | 
   | 
$  | 
120,675  | 
 
  | 
 
Gain on sales of real estate, net  | 
   | 
9,890  | 
 
  | 
   | 
40,986  | 
 
  | 
   | 
68,047  | 
 
  | 
 
Preferred unit distributions  | 
   | 
(6,879  | 
)  | 
   | 
(14,957  | 
)  | 
   | 
(14,870  | 
)  | 
 
Issuance costs associated with redeemed preferred units  | 
   | 
(6,847  | 
)  | 
   | 
(17  | 
)  | 
   | 
—  | 
 
  | 
 
Income from continuing operations attributable to noncontrolling interests  | 
   | 
(3,646  | 
)  | 
   | 
(3,715  | 
)  | 
   | 
(3,523  | 
)  | 
 
Income from continuing operations attributable to share-based compensation awards  | 
   | 
(449  | 
)  | 
   | 
(419  | 
)  | 
   | 
(706  | 
)  | 
 
Numerator for basic and diluted EPU from continuing operations attributable to COPLP common unitholders  | 
   | 
$  | 
58,512  | 
 
  | 
   | 
$  | 
(3,453  | 
)  | 
   | 
$  | 
169,623  | 
 
  | 
 
Discontinued operations  | 
   | 
—  | 
 
  | 
   | 
—  | 
 
  | 
   | 
156  | 
 
  | 
 
Discontinued operations attributable to noncontrolling interests  | 
   | 
—  | 
 
  | 
   | 
—  | 
 
  | 
   | 
3  | 
 
  | 
 
Numerator for basic and diluted EPU on net income (loss) attributable to COPLP common unitholders  | 
   | 
$  | 
58,512  | 
 
  | 
   | 
$  | 
(3,453  | 
)  | 
   | 
$  | 
169,782  | 
 
  | 
 
Denominator (all weighted averages):  | 
   | 
   | 
   | 
   | 
   | 
   | 
 
Denominator for basic EPU (common units)  | 
   | 
102,331  | 
 
  | 
   | 
98,135  | 
 
  | 
   | 
97,606  | 
 
  | 
 
Dilutive effect of forward equity sale agreements and share-based compensation awards  | 
   | 
186  | 
 
  | 
   | 
—  | 
 
  | 
   | 
61  | 
 
  | 
 
Denominator for diluted EPU (common units)  | 
   | 
102,517  | 
 
  | 
   | 
98,135  | 
 
  | 
   | 
97,667  | 
 
  | 
 
Basic EPU:  | 
   | 
   | 
   | 
   | 
   | 
   | 
 
Income (loss) from continuing operations attributable to COPLP common unitholders  | 
   | 
$  | 
0.57  | 
 
  | 
   | 
$  | 
(0.04  | 
)  | 
   | 
$  | 
1.74  | 
 
  | 
 
Net income (loss) attributable to COPLP common unitholders  | 
   | 
$  | 
0.57  | 
 
  | 
   | 
$  | 
(0.04  | 
)  | 
   | 
$  | 
1.74  | 
 
  | 
 
Diluted EPU:  | 
   | 
   | 
   | 
   | 
   | 
   | 
 
Income (loss) from continuing operations attributable to COPLP common unitholders  | 
   | 
$  | 
0.57  | 
 
  | 
   | 
$  | 
(0.04  | 
)  | 
   | 
$  | 
1.74  | 
 
  | 
 
Net income (loss) attributable to COPLP common unitholders  | 
   | 
$  | 
0.57  | 
 
  | 
   | 
$  | 
(0.04  | 
)  | 
   | 
$  | 
1.74  | 
 
  | 
 
  
  
 | 
| Schedule of securities excluded from computation of diluted earnings per share | 
Our diluted EPU computations do not include the effects of the following securities since the conversions of such securities would increase diluted EPU for the respective periods (in thousands):
 |  
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 | 
 
   | 
   | 
Weighted Average Units Excluded from Denominator for the Years Ended December 31,  | 
 
   | 
   | 
2017  | 
   | 
2016  | 
   | 
2015  | 
 
Conversion of Series I preferred units  | 
   | 
176  | 
 
  | 
   | 
176  | 
 
  | 
   | 
176  | 
 
  | 
 
Conversion of Series K preferred units  | 
   | 
—  | 
 
  | 
   | 
434  | 
 
  | 
   | 
434  | 
 
  | 
 
  
  
 |