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Equity - COPT and Subsidiaries (Details) - USD ($) |  |  |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | ---
Dec. 27, 2017 | Nov. 02, 2017 | Jun. 27, 2017 | Jan. 21, 2017 | Dec. 31, 2017 | Sep. 30, 2016 | Oct. 31, 2012 | Dec. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Stock redeemed or called during period value |  |  |  |  |  |  |  |  | $ 172,500,000 | $ 26,583,000 |
Common Shares |  |  |  |  |  |  |  |  |  |  |
Common shares issued under forward equity sale agreements |  |  |  |  |  |  |  |  | $ 49,944,000 |  |
Number of common shares for each converted common unit (in shares) |  |  |  |  |  |  |  |  | 1 |  |
Number of operating partnerships units converted into common shares (in units) |  |  |  |  |  |  |  |  | 339,513 | 87,000 | 160,160
Dividends declared per common share (in dollars per share) |  |  |  |  |  |  |  |  | $ 1.1 | $ 1.10 | $ 1.10
Common Stock Issued to Public Under At-the-Market Program [Member] |  |  |  |  |  |  |  |  |  |  |
Common Shares |  |  |  |  |  |  |  |  |  |  |
At-market-stock, offering program established, aggregate value |  |  |  |  |  | $ 200,000,000 | $ 150,000,000.0 |  |  |  |
Shares issued to the public |  |  |  |  |  |  |  | 3,720,000 | 591,042.000 | 3,720,000 | 890,241
Net proceeds from shares issued |  |  |  |  |  |  |  | $ 109,100,000 | $ 19,700,000 |  | $ 26,600,000
Payments of stock issuance costs |  |  |  |  |  |  |  | $ 900,000 | 300,000 | $ 900,000 | $ 400,000
At-market stock, offering program established, remaining capacity |  |  |  |  | $ 70,000,000 |  |  |  | $ 70,000,000 |  |
Issuance of stock, weighted average price per share (in dollars per share) |  |  |  |  | $ 33.84 |  |  | $ 29.56 | $ 33.84 | $ 29.56 | $ 30.29
Common Shares [Member] |  |  |  |  |  |  |  |  |  |  |
Common Shares |  |  |  |  |  |  |  |  |  |  |
Common shares issued under forward equity sale agreements |  |  |  |  |  |  |  |  | $ 17,000 |  |
Forward Equity Sale Agreement [Member] |  |  |  |  |  |  |  |  |  |  |
Common Shares |  |  |  |  |  |  |  |  |  |  |
Shares issued to the public | 1,700,000 |  |  |  | 1,700,000 |  |  |  |  |  |
Forward equity sale agreements number of shares available for issuance |  | 9,200,000 |  |  |  |  |  |  |  |  |
Net proceeds from issue of shares after underwriter discounts but before offering expenses |  | $ 285,200,000 |  |  |  |  |  |  |  |  |
Initial Gross Offering Price Per Share |  | $ 31.00 |  |  |  |  |  |  |  |  |
Common shares issued under forward equity sale agreements | $ 50,000,000 |  |  |  | $ 50,000,000 |  |  |  |  |  |
Preferred Shares [Member] |  |  |  |  |  |  |  |  |  |  |
Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Number of preferred shares authorized |  |  |  |  | 25,000,000 |  |  |  | 25,000,000 |  |
Number of preferred shares of beneficial interest authorized (in dollars per share) |  |  |  |  | $ 0.01 |  |  |  | $ 0.01 |  |
Series K Preferred Shares [Member] |  |  |  |  |  |  |  |  |  |  |
Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Annual dividend yield |  |  |  | 5.60% |  |  |  |  |  |  |
Issuance costs associated with redeemed preferred shares |  |  |  |  |  |  |  | $ 17,000 |  |  |
Series L [Member] |  |  |  |  |  |  |  |  |  |  |
Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Annual dividend yield |  |  | 7.375% |  |  |  |  |  |  |  |
Issuance costs associated with redeemed preferred shares |  |  |  |  |  |  |  |  | $ 6,800,000 |  |

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