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Summary of Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2017
Accounting Policies [Abstract] |
Schedule of amortization of intangible assets and deferred revenue | We amortize the intangible assets and deferred revenue on real estate acquisitions discussed above as follows:
Asset Type |  | Amortization Period
Above- and below-market leases |  | Related lease terms
In-place lease value |  | Related lease terms
Tenant relationship value |  | Estimated period of time that tenant will lease space in property
Above- and below-market cost arrangements |  | Term of arrangements
Schedule of the estimated useful lives of fixed assets | We depreciate our fixed assets using the straight-line method over their estimated useful lives as follows:
|  | Estimated Useful Lives
Buildings and building improvements |  | 10-40 years
Land improvements |  | 10-20 years
Tenant improvements on operating properties |  | Related lease term
Equipment and personal property |  | 3-10 years
Schedule of characterization of dividends declared on common and preferred shares | The characterization of dividends paid on COPT’s common and preferred shares during each of the last three years was as follows:
|  | Common Shares |  | Preferred Shares
|  | For the Years Ended December 31, |  | For the Years Ended December 31,
|  | 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Ordinary income |  | 86.5 | % |  | 48.0 | % |  | 38.3 | % |  | 100.0 | % |  | 100 | % |  | 38.3 | %
Long-term capital gain |  | 0.0 | % |  | 0.0 | % |  | 61.7 | % |  | 0.0 | % |  | 0.0 | % |  | 61.7 | %
Return of capital |  | 13.5 | % |  | 52.0 | % |  | 0.0 | % |  | 0.0 | % |  | 0.0 | % |  | 0.0 | %

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- Definition Tabular disclosure of amortization periods of intangible assets and deferred revenue, by major class. + References No definition available. + Details
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- Definition Tabular disclosure of physical assets used in the normal conduct of business and not intended for resale. Includes, but is not limited to, balances by class of assets, depreciation and depletion expense and method used, including composite depreciation, and accumulated deprecation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(13)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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