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Equity - COPLP and Subsidiaries (Details) - USD ($) $ / shares in Units, $ in Thousands |  |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
Dec. 27, 2017 | Jun. 27, 2017 | Jan. 21, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Preferred Stock |  |  |  |  |  |  |  |
Stock redeemed or called during period value |  |  |  |  |  | $ 172,500 | $ 26,583 |
Preferred units in COPLP |  |  |  | $ 8,800 | $ 8,800 | 8,800 | $ 8,800 |
Common Shares |  |  |  |  |  |  |  |
Common shares issued under forward equity sale agreements |  |  |  |  |  | $ 49,944 |  |
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 |  |  |  |  |  |  |  |
Shares issued to the public |  |  |  |  | 3,720,000 | 591,042.000 | 3,720,000 | 890,241
Issuance of stock, weighted average price per share (in dollars per share) |  |  |  | $ 33.84 | $ 29.56 | $ 33.84 | $ 29.56 | $ 30.29
Net proceeds from issue of shares after underwriter discounts but before offering expenses |  |  |  |  |  | $ 19,700 | $ 109,100 | $ 26,600
Payments of stock issuance costs |  |  |  |  | $ 900 | 300 | 900 | $ 400
Forward Equity Sale Agreement [Member] |  |  |  |  |  |  |  |
Common Shares |  |  |  |  |  |  |  |
Shares issued to the public | 1,700,000 |  |  | 1,700,000 |  |  |  |
Common shares issued under forward equity sale agreements | $ 50,000 |  |  | $ 50,000 |  |  |  |
Series K Preferred Shares [Member] |  |  |  |  |  |  |  |
Preferred Stock |  |  |  |  |  |  |  |
Annual dividend yield |  |  | 5.60% |  |  |  |  |
Issuance costs associated with redeemed preferred shares |  |  |  |  | 17 |  |  |
Series L [Member] |  |  |  |  |  |  |  |
Preferred Stock |  |  |  |  |  |  |  |
Annual dividend yield |  | 7.375% |  |  |  |  |  |
Issuance costs associated with redeemed preferred shares |  |  |  |  |  | 6,800 |  |
Corporate Office Properties, L.P. [Member] |  |  |  |  |  |  |  |
Preferred Stock |  |  |  |  |  |  |  |
Stock redeemed or called during period value |  |  |  |  |  | 172,500 | $ 26,583 |
Common Shares |  |  |  |  |  |  |  |
Common shares issued under forward equity sale agreements |  |  |  |  |  | $ 49,944 |  |
Number of common shares for each converted common unit (in shares) |  |  |  |  |  | 1 |  |
Number of operating partnerships units converted into common shares (in units) |  |  |  |  |  | 339,513.000 | 87,000 | 160,160.00
Dividends declared per common share (in dollars per share) |  |  |  |  |  | $ 1.1 | $ 1.10 | $ 1.10
Corporate Office Properties, L.P. [Member] | Common Units [Member] |  |  |  |  |  |  |  |
Common Shares |  |  |  |  |  |  |  |
Percentage ownership in operating partnership |  |  |  |  |  | 96.90% | 96.50% |
Corporate Office Properties, L.P. [Member] | Conversion of Series I preferred units [Member] |  |  |  |  |  |  |  |
Preferred Stock |  |  |  |  |  |  |  |
Annual dividend yield |  |  |  |  |  | 7.50% |  |
Limited partners' capital account, units issued |  |  |  | 352,000 |  | 352,000 |  |
Preferred units in COPLP |  |  |  | $ 8,800 |  | $ 8,800 |  |
Preferred stock, liquidation preference per share |  |  |  | $ 25 |  | $ 25 |  |
Annual cumulative preferred return increment frequency |  |  |  |  |  | 5 years |  |
Common units conversion basis units issuable |  |  |  |  |  | 0.5 |  |
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 |  |  |  |  |
Issuance costs associated with redeemed preferred shares |  |  |  |  | $ 17 |  |  |
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 |  |  |  |  |  |
Issuance costs associated with redeemed preferred shares |  |  |  |  |  | $ 6,800 |  |

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