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Properties, net (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
--- | --- | --- | --- | ---
Properties |  |  |  |  |
Gross | $ 4,014,336 us-gaap_RealEstateGrossAtCarryingValue | [1],[2] | $ 3,811,950 us-gaap_RealEstateGrossAtCarryingValue | $ 3,859,960 us-gaap_RealEstateGrossAtCarryingValue | $ 4,038,932 us-gaap_RealEstateGrossAtCarryingValue
Operating properties, net | 2,751,488 us-gaap_RentalProperties |  | 2,702,693 us-gaap_RentalProperties |  |
Operating properties, net |  |  |  |  |
Properties |  |  |  |  |
Less: accumulated depreciation | (703,083)us-gaap_RealEstateInvestmentPropertyAccumulatedDepreciation / ofc_RealEstatePropertiesStatusAxis = ofc_OperatingPropertiesMember |  | (597,649)us-gaap_RealEstateInvestmentPropertyAccumulatedDepreciation / ofc_RealEstatePropertiesStatusAxis = ofc_OperatingPropertiesMember |  |
Operating properties, net | Land |  |  |  |  |
Properties |  |  |  |  |
Gross | 439,355 us-gaap_RealEstateGrossAtCarryingValue / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandMember / ofc_RealEstatePropertiesStatusAxis = ofc_OperatingPropertiesMember |  | 430,472 us-gaap_RealEstateGrossAtCarryingValue / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandMember / ofc_RealEstatePropertiesStatusAxis = ofc_OperatingPropertiesMember |  |
Operating properties, net | Buildings and improvements |  |  |  |  |
Properties |  |  |  |  |
Gross | $ 3,015,216 us-gaap_RealEstateGrossAtCarryingValue / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_BuildingAndBuildingImprovementsMember / ofc_RealEstatePropertiesStatusAxis = ofc_OperatingPropertiesMember |  | $ 2,869,870 us-gaap_RealEstateGrossAtCarryingValue / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_BuildingAndBuildingImprovementsMember / ofc_RealEstatePropertiesStatusAxis = ofc_OperatingPropertiesMember |  |
[1] | As discussed in Note 3 to our Consolidated Financial Statements, we recognized impairment losses of $1.4 million primarily in connection with certain of our operating properties in the Greater Baltimore region that were disposed in the current period.
[2] | The aggregate cost of these assets for Federal income tax purposes was approximately $3.4 billion at December 31, 2014.

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- Definition Amount, before accumulated depreciation, of real estate investments for entities with a substantial portion of business acquiring and holding investment real estate. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 10 -Section S99 -Paragraph 6 -Subparagraph (SX 210.5-04.(c) Schedule III) -URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 28 -Article 12 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph c -Subparagraph Schedule III -Article 5 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 970 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-28.4) -URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024 + Details
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- Definition The cumulative amount of depreciation for real estate property held for investment purposes. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(3)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Article 7 + Details
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- Definition Carrying amount of income producing properties held for rental. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(d)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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