Share-Based Compensation and Other Compensation Matters (Tables)
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12 Months Ended | 
Dec. 31, 2017  | 
| Disclosure of Compensation Related Costs, Share-based Payments [Abstract] | 
 
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| Reporting for share-based compensation expense | 
The table below sets forth our reporting for share based compensation cost (in thousands):
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   | 
   | 
 For the Years Ended December 31,  | 
 
   | 
   | 
2017  | 
   | 
2016  | 
   | 
2015  | 
 
General, administrative and leasing expenses  | 
   | 
$  | 
4,649  | 
 
  | 
   | 
$  | 
5,816  | 
 
  | 
   | 
$  | 
5,574  | 
 
  | 
 
Property operating expenses  | 
   | 
966  | 
 
  | 
   | 
1,027  | 
 
  | 
   | 
1,000  | 
 
  | 
 
Capitalized to development activities  | 
   | 
480  | 
 
  | 
   | 
610  | 
 
  | 
   | 
824  | 
 
  | 
 
Share-based compensation cost  | 
   | 
$  | 
6,095  | 
 
  | 
   | 
$  | 
7,453  | 
 
  | 
   | 
$  | 
7,398  | 
 
  | 
 
  
  
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| Summary of restricted share transactions under the entity's share-based compensation plans | 
The following table summarizes restricted shares under the share-based compensation plans for 2015, 2016 and 2017:
 
 
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   | 
   | 
 Shares  | 
   | 
Weighted Average Grant Date Fair Value  | 
 
Unvested as of December 31, 2014  | 
   | 
390,507  | 
 
  | 
   | 
$  | 
26.19  | 
 
  | 
 
Granted  | 
   | 
201,024  | 
 
  | 
   | 
28.69  | 
 
  | 
 
Forfeited  | 
   | 
(10,550  | 
)  | 
   | 
26.05  | 
 
  | 
 
Vested  | 
   | 
(202,781  | 
)  | 
   | 
26.07  | 
 
  | 
 
Unvested as of December 31, 2015  | 
   | 
378,200  | 
 
  | 
   | 
27.58  | 
 
  | 
 
Granted  | 
   | 
231,937  | 
 
  | 
   | 
24.77  | 
 
  | 
 
Forfeited  | 
   | 
(22,907  | 
)  | 
   | 
25.31  | 
 
  | 
 
Vested  | 
   | 
(215,983  | 
)  | 
   | 
27.19  | 
 
  | 
 
Unvested as of December 31, 2016  | 
   | 
371,247  | 
 
  | 
   | 
26.20  | 
 
  | 
 
Granted  | 
   | 
239,479  | 
 
  | 
   | 
33.84  | 
 
  | 
 
Forfeited  | 
   | 
(27,056  | 
)  | 
   | 
27.80  | 
 
  | 
 
Vested  | 
   | 
(158,044  | 
)  | 
   | 
26.27  | 
 
  | 
 
Unvested as of December 31, 2017  | 
   | 
425,626  | 
 
  | 
   | 
$  | 
30.37  | 
 
  | 
 
Unvested shares as of December 31, 2017 that are expected to vest  | 
   | 
402,870  | 
 
  | 
   | 
$  | 
30.31  | 
 
  | 
 
  
 
 
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| Schedule of PSU's Granted | 
We made the following grants of PSUs to executives from 2013 through 2017 (dollars in thousands):
 
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Grant Date  | 
   | 
Number of PSUs Granted  | 
   | 
Performance Period Commencement Date  | 
   | 
Performance Period End Date  | 
   | 
Grant Date Fair Value  | 
   | 
Number of PSUs Outstanding as of December 31, 2017  | 
 
3/1/2013  | 
   | 
69,579  | 
 
  | 
   | 
1/1/2013  | 
   | 
12/31/2015  | 
   | 
$  | 
1,867  | 
 
  | 
   | 
—  | 
 
  | 
 
3/6/2014  | 
   | 
49,103  | 
 
  | 
   | 
1/1/2014  | 
   | 
12/31/2016  | 
   | 
$  | 
1,723  | 
 
  | 
   | 
—  | 
 
  | 
 
3/5/2015  | 
   | 
45,656  | 
 
  | 
   | 
1/1/2015  | 
   | 
12/31/2017  | 
   | 
$  | 
1,678  | 
 
  | 
   | 
15,767  | 
 
  | 
 
3/1/2016  | 
   | 
26,299  | 
 
  | 
   | 
1/1/2016  | 
   | 
12/31/2018  | 
   | 
$  | 
1,000  | 
 
  | 
   | 
24,850  | 
 
  | 
 
1/1/2017  | 
   | 
39,351  | 
 
  | 
   | 
1/1/2017  | 
   | 
12/31/2019  | 
   | 
$  | 
1,400  | 
 
  | 
   | 
39,351  | 
 
  | 
 
  
 
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| Schedule of payouts for defined performance under performance-based awards of share-based compensation | 
The number of PSUs earned (“earned PSUs”) at the end of the performance period will be determined based on the percentile rank of COPT’s total shareholder return relative to a peer group of companies, as set forth in the following schedule: 
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Percentile Rank  | 
   | 
Earned PSUs Payout %  | 
 
75th or greater  | 
   | 
200% of PSUs granted  | 
 
50th or greater  | 
   | 
100% of PSUs granted  | 
 
25th  | 
   | 
50% of PSUs granted  | 
 
Below 25th  | 
   | 
0% of PSUs granted  | 
 
  
 
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| Schedule of assumptions used in Monte Carlo models for PSUs | 
The grant date fair value and certain of the assumptions used in the Monte Carlo models for the PSUs granted in 2015, 2016 and 2017 are set forth below:
 
 
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Grant Date  | 
   | 
Grant Date Fair Value  | 
   | 
Baseline Common Share Value  | 
   | 
Expected Volatility of Common Shares  | 
   | 
Risk-free Interest Rate  | 
 
3/5/2015  | 
   | 
$  | 
36.76  | 
 
  | 
   | 
$  | 
29.28  | 
 
  | 
   | 
19.9  | 
%  | 
   | 
0.99  | 
%  | 
 
3/1/2016  | 
   | 
$  | 
38.21  | 
 
  | 
   | 
$  | 
23.90  | 
 
  | 
   | 
20.4  | 
%  | 
   | 
0.96  | 
%  | 
 
1/1/2017  | 
   | 
$  | 
38.43  | 
 
  | 
   | 
$  | 
31.22  | 
 
  | 
   | 
19.0  | 
%  | 
   | 
1.47  | 
%  | 
 
  
 
 
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| Deferred share awards | 
We made the following grants of deferred share awards to nonemployee members of our Board of Trustees in 2015, 2016 and 2017 (dollars in thousands, except per share amounts): 
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Year of Grant  | 
   | 
Number of Deferred Share Awards Granted  | 
   | 
Aggregate Grant Date Fair Value  | 
   | 
Grant Date Fair Value Per Share  | 
 
2015  | 
   | 
24,056  | 
 
  | 
   | 
$  | 
642  | 
 
  | 
   | 
$  | 
26.70  | 
 
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2016  | 
   | 
24,944  | 
 
  | 
   | 
$  | 
671  | 
 
  | 
   | 
$  | 
26.89  | 
 
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2017  | 
   | 
10,032  | 
 
  | 
   | 
$  | 
326  | 
 
  | 
   | 
$  | 
32.47  | 
 
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| Deferred share awards vested activity | 
We issued the following common shares in settlement of deferred shares in 2015, 2016 and 2017 (dollars in thousands, except per share amounts): 
 
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 For the Years Ended December 31,  | 
 
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2017  | 
   | 
2016  | 
   | 
2015  | 
 
Number of common shares issued  | 
   | 
15,590  | 
 
  | 
   | 
12,028  | 
 
  | 
   | 
15,485  | 
 
  | 
 
Grant date fair value  | 
   | 
$  | 
26.89  | 
 
  | 
   | 
$  | 
26.70  | 
 
  | 
   | 
$  | 
26.77  | 
 
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Aggregate intrinsic value  | 
   | 
$  | 
508  | 
 
  | 
   | 
$  | 
322  | 
 
  | 
   | 
$  | 
413  | 
 
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| Summary of stock option transactions under the entity's share-based compensation plans | 
The table below sets forth information regarding our outstanding options as of the following dates (dollars in thousands, except per share data): 
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 Options Outstanding and Exercisable  | 
   | 
Weighted Average Exercise Price Per Share  | 
   | 
 Weighted Average Remaining Contractual Term  
(in Years) 
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   | 
Aggregate Intrinsic Value  | 
 
December 31, 2014  | 
   | 
559,736  | 
 
  | 
   | 
$39.60  | 
   | 
2  | 
   | 
$  | 
167  | 
 
  | 
 
December 31, 2015  | 
   | 
425,347  | 
 
  | 
   | 
$42.75  | 
   | 
1  | 
   | 
$  | 
—  | 
 
  | 
 
December 31, 2016  | 
   | 
201,100  | 
 
  | 
   | 
$43.35  | 
   | 
1  | 
   | 
$  | 
31  | 
 
  | 
 
December 31, 2017  | 
   | 
60,000  | 
 
  | 
   | 
$35.17  | 
   | 
1  | 
   | 
$  | 
—  | 
 
  | 
 
  
 
 
  
 
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