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Equity - COPT and Subsidiaries (Details) - USD ($) |  |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
Nov. 02, 2017 | Jun. 27, 2017 | Jan. 21, 2017 | Nov. 30, 2018 | Sep. 30, 2016 | Dec. 31, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Preferred Stock |  |  |  |  |  |  |  |  |
Stock redeemed or called during period value |  |  |  |  |  |  |  | $ 172,500,000 | $ 26,583,000
Common Shares |  |  |  |  |  |  |  |  |
Number of common shares for each converted common unit (in shares) |  |  |  |  |  |  | 1 |  |
Number of operating partnerships units converted into common shares (in units) |  |  |  |  |  |  | 1,904,615 | 339,513 | 87,000
Dividends declared per common share (in dollars per share) |  |  |  |  |  |  | $ 1.1 | $ 1.10 | $ 1.10
Preferred Shares [Member] |  |  |  |  |  |  |  |  |
Preferred Stock |  |  |  |  |  |  |  |  |
Number of preferred shares authorized |  |  |  |  |  |  | 25,000,000 |  |
Number of preferred shares of beneficial interest authorized (in dollars per share) |  |  |  |  |  |  | $ 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 |
2016 ATM Program [Member] |  |  |  |  |  |  |  |  |
Common Shares |  |  |  |  |  |  |  |  |
Shares issued to the public |  |  |  |  |  |  | 992,000 | 591,000 | 3,700,000
Issuance of stock, weighted average price per share (in dollars per share) |  |  |  |  |  | $ 29.56 | $ 30.46 | $ 33.84 | $ 29.56
Payments of stock issuance costs |  |  |  |  |  |  | $ 500,000 | $ 300,000 | $ 900,000
Net proceeds from issuance of shares after underwriting discounts and commissions but before offering expenses |  |  |  |  |  |  | $ 29,800,000 | $ 19,700,000 | $ 109,100,000
2016 ATM Program [Member] | Common Shares [Member] |  |  |  |  |  |  |  |  |
Common Shares |  |  |  |  |  |  |  |  |
At-market-stock, offering program established, aggregate value |  |  |  |  | $ 200,000,000 |  |  |  |
Shares issued to the public |  |  |  |  |  | 3,700,000 | 992,000 | 591,000 |
Issuance of stock, weighted average price per share (in dollars per share) |  |  |  |  |  | $ 29.56 | $ 30.46 | $ 33.84 | $ 29.56
Payments of stock issuance costs |  |  |  |  |  | $ 900,000 | $ 500,000 | $ 300,000 |
Net proceeds from issuance of shares after underwriting discounts and commissions but before offering expenses |  |  |  |  |  | 109,100,000 | $ 29,800,000 | $ 19,700,000 |
2018 ATM Program [Member] | Common Shares [Member] |  |  |  |  |  |  |  |  |
Common Shares |  |  |  |  |  |  |  |  |
At-market-stock, offering program established, aggregate value |  |  |  | $ 300,000,000 |  |  |  |  |
Forward Equity Sale Agreement [Member] | Common Shares [Member] |  |  |  |  |  |  |  |  |
Common Shares |  |  |  |  |  |  |  |  |
Forward Equity Sale Agreement, Offering Program Established, Aggregate Shares | 9,200,000 |  |  |  |  |  |  |  |
Shares issued to the public |  |  |  |  |  |  | 5,900,000 | 1,700,000 |
Net proceeds from issuance of shares before underwriting discounts, commission and offering expenses | $ 285,200,000 |  |  |  |  |  |  |  |
Initial gross offering price per share (in dollars per share) | $ 31.00 |  |  |  |  |  |  |  |
Net proceeds from issuance of shares after underwriting discounts and commissions but before offering expenses |  |  |  |  |  |  | $ 172,500,000 | $ 50,000,000 |
Remaining shares under agreements |  |  |  |  |  |  | 1,600,000 |  |
Settlement value of remaining capacity |  |  |  |  |  |  | $ 46,400,000 |  |
Corporate Office Properties, L.P. [Member] |  |  |  |  |  |  |  |  |
Preferred Stock |  |  |  |  |  |  |  |  |
Stock redeemed or called during period value |  |  |  |  |  |  |  | $ 172,500,000 | $ 26,583,000
Common Shares |  |  |  |  |  |  |  |  |
Number of common shares for each converted common unit (in shares) |  |  |  |  |  |  | 1 |  |
Number of operating partnerships units converted into common shares (in units) |  |  |  |  |  |  | 1,900,000 | 339,513.000 | 87,000
Dividends declared per common share (in dollars per share) |  |  |  |  |  |  | $ 1.1 | $ 1.10 | $ 1.10
Corporate Office Properties, L.P. [Member] | Series K Preferred Shares [Member] |  |  |  |  |  |  |  |  |
Preferred Stock |  |  |  |  |  |  |  |  |
Annual dividend yield |  |  | 5.60% |  |  |  |  |  |
Preferred stock, redemption price per share (in dollars per share) |  |  | $ 50.00 |  |  |  |  |  |
Stock redeemed or called during period value |  |  | $ 26,600,000 |  |  |  |  |  |
Issuance costs associated with redeemed preferred shares |  |  |  |  |  | $ 17,000 |  |  |
Corporate Office Properties, L.P. [Member] | Series L [Member] |  |  |  |  |  |  |  |  |
Preferred Stock |  |  |  |  |  |  |  |  |
Annual dividend yield |  | 7.375% |  |  |  |  |  |  |
Preferred stock, redemption price per share (in dollars per share) |  | $ 25.00 |  |  |  |  |  |  |
Stock redeemed or called during period value |  | $ 172,500,000 |  |  |  |  |  |  |
Issuance costs associated with redeemed preferred shares |  |  |  |  |  |  |  | $ 6,800,000 |

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