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Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2016 | Sep. 30, 2018 | Jan. 01, 2018 | Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Minimum rental payments due |  | $ 88,326 |  |
Ground Leases |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Minimum rental payments due |  | $ 87,500 |  |
Accounting Standards Update 2017-05 |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Effect of change on net income | $ 18,000 |  |  |
Accumulated Other Comprehensive Income (Loss) |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Cumulative effect of accounting change for adoption of hedge accounting guidance |  |  |  | $ 276
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2017-12 |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Cumulative effect of accounting change for adoption of hedge accounting guidance |  |  | $ 276 |
Accumulated Other Comprehensive Income (Loss) | Corporate Office Properties, L.P. |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Cumulative effect of accounting change for adoption of hedge accounting guidance |  |  |  | 276
Accumulated Other Comprehensive Income (Loss) | Corporate Office Properties, L.P. | Accounting Standards Update 2017-12 |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Cumulative effect of accounting change for adoption of hedge accounting guidance |  |  | 276 |
Cumulative Distributions in Excess of Net Income |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Cumulative effect of accounting change for adoption of hedge accounting guidance |  |  |  | (276)
Cumulative Distributions in Excess of Net Income | Accounting Standards Update 2017-12 |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Cumulative effect of accounting change for adoption of hedge accounting guidance |  |  | (276) |
Common Shares | Corporate Office Properties, L.P. |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Cumulative effect of accounting change for adoption of hedge accounting guidance |  |  |  | $ (276)
Common Shares | Corporate Office Properties, L.P. | Accounting Standards Update 2017-12 |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Cumulative effect of accounting change for adoption of hedge accounting guidance |  |  | $ (276) |

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