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Share-Based Compensation (Tables) | 9 Months Ended
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Sep. 30, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
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:
Percentile Rank |  | Earned PSUs Payout %
75th or greater |  | 200% of PSUs granted
50th |  | 100% of PSUs granted
25th |  | 50% of PSUs granted
Below 25th |  | 0% of PSUs granted

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