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Equity - COPT and Subsidiaries (Details) - USD ($) | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Nov. 30, 2018 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Nov. 02, 2017
Common Shares |  |  |  |  |
Number of common shares for each converted common unit (in shares) |  | 1 | 1 | 1 |
Number of operating partnerships units converted into common shares (in units) |  | 14,009 | 105,039 | 1,904,615 |
Dividends declared per common share (in dollars per share) |  | $ 1.10 | $ 1.10 | $ 1.10 |
Preferred Shares |  |  |  |  |
Preferred Stock |  |  |  |  |
Number of preferred shares authorized |  | 25,000,000.0 |  |  |
Number of preferred shares of beneficial interest authorized (in dollars per share) |  | $ 0.01 |  |  |
2018 ATM Program | Common Shares |  |  |  |  |
Common Shares |  |  |  |  |
At-market-stock, offering program established, aggregate value | $ 300,000,000.0 |  |  |  |
Shares issued to the public (in shares) |  | 0 |  |  |
Forward Equity Sale Agreement | Common Shares |  |  |  |  |
Common Shares |  |  |  |  |
Shares issued to the public (in shares) |  |  | 1,600,000 | 5,900,000 |
Net proceeds from issuance of shares after underwriting discounts and commissions but before offering expenses |  |  | $ 46,500,000 | $ 172,500,000 |
Initial gross offering price per share (in dollars per share) |  |  |  |  | $ 31.00
2016 ATM Program | Common Shares |  |  |  |  |
Common Shares |  |  |  |  |
Shares issued to the public (in shares) |  |  |  | 992,000 |
Net proceeds from issuance of shares after underwriting discounts and commissions but before offering expenses |  |  |  | $ 29,800,000 |
Issuance of stock, weighted average price per share (in dollars per share) |  |  |  | $ 30.46 |
Payments of stock issuance costs |  |  |  | $ 500,000 |

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