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Equity (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Class of Stock [Line Items] |  |  |  |
Dividends declared per share (in usd per share) | $ 0.2750 | $ 0.275 | $ 0.825 | $ 0.825
Conversion of common units to common shares (in units/shares) |  |  | 1,895,627 | 337,000
Common Shares | Forward Equity Sale Agreement |  |  |  |
Class of Stock [Line Items] |  |  |  |
Number of shares issued |  |  | 4,500,000 |
Proceeds from common shares issued under forward equity sale agreements |  |  | $ 132.5 |
Remaining capacity under forward equity sale agreements | 3,000,000 |  | 3,000,000 |
Common Shares | Common Stock Issued to Public Under At-the-Market Program |  |  |  |
Class of Stock [Line Items] |  |  |  |
Number of shares issued |  |  | 991,664 |
Weighted average price per share (in dollars per share) | $ 30.46 |  | $ 30.46 |
Proceeds from Issuance of Common Stock, Net of Issuance Costs |  |  | $ 29.8 |
Payments of commissions to sales agents |  |  | 0.5 |
At-the-market stock, offering program established, remaining capacity | $ 39.8 |  | $ 39.8 |
Corporate Office Properties, L.P. |  |  |  |
Class of Stock [Line Items] |  |  |  |
Dividends declared per share (in usd per share) | $ 0.275 | $ 0.275 | $ 0.825 | $ 0.825

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