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Fair Value Measurements (Details 2) - Fair value measurement on a recurring basis [Member] - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Assets: |  |
Interest rate derivatives | $ 3,073 | $ 158
Assets | 7,689 | 5,595
Liabilities: |  |
Deferred compensation plan liability | 4,616 | 5,437
Interest rate derivatives |  | 1,572
Redeemable preferred shares of beneficial interest |  | 26,583
Liabilities |  | 33,592
Mutual funds [Member] |  |
Assets: |  |
Marketable securities in deferred compensation plan | 4,547 | 5,346
Other [Member] |  |
Assets: |  |
Marketable securities in deferred compensation plan | 69 | 91
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |
Assets: |  |
Assets | 4,616 | 5,437
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Mutual funds [Member] |  |
Assets: |  |
Marketable securities in deferred compensation plan | 4,547 | 5,346
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other [Member] |  |
Assets: |  |
Marketable securities in deferred compensation plan | 69 | 91
Significant Other Observable Inputs (Level 2) [Member] |  |
Assets: |  |
Interest rate derivatives | 3,073 | 158
Assets | 3,073 | 158
Liabilities: |  |
Deferred compensation plan liability | 4,616 | 5,437
Interest rate derivatives |  | 1,572
Redeemable preferred shares of beneficial interest |  | 26,583
Liabilities |  | 33,592
Corporate Office Properties, L.P. [Member] |  |
Assets: |  |
Interest rate derivatives | 3,073 | 158
Liabilities: |  |
Interest rate derivatives |  | 1,572
Redeemable preferred shares of beneficial interest |  | 26,583
Liabilities |  | 28,155
Corporate Office Properties, L.P. [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Assets: |  |
Interest rate derivatives | $ 3,073 | 158
Liabilities: |  |
Interest rate derivatives |  | 1,572
Redeemable preferred shares of beneficial interest |  | 26,583
Liabilities |  | $ 28,155

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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