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Fair Value Measurements (Additional Information) (Details) (USD $) | 12 Months Ended
--- | ---
Dec. 31, 2014 Impaired_Asset_And_Liabilty | Dec. 31, 2013 Impaired_Asset_And_Liabilty | Dec. 31, 2012 Impaired_Asset_And_Liabilty
Financial Assets and Liabilities Fair Value Measurement |  |  |
Number of non-financial assets and liabilities impaired | 0 cog_NumberOfNonFinancialAssetsAndLiabilitiesImpaired | 0 cog_NumberOfNonFinancialAssetsAndLiabilitiesImpaired | 0 cog_NumberOfNonFinancialAssetsAndLiabilitiesImpaired
Fair value assets Level 1 to Level 2 transfers amount | $ 0 us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount | $ 0 us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount | $ 0 us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount
Fair value liabilities Level 1 to Level 2 transfers amount | 0 us-gaap_FairValueLiabilitiesLevel1ToLevel2TransfersAmount | 0 us-gaap_FairValueLiabilitiesLevel1ToLevel2TransfersAmount | 0 us-gaap_FairValueLiabilitiesLevel1ToLevel2TransfersAmount
Fair value assets Level 2 to Level 1 transfers amount | 0 us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount | 0 us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount | 0 us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount
Fair value liabilities Level 2 to Level 1 transfers amount | 0 us-gaap_FairValueLiabilitiesLevel2ToLevel1TransfersAmount | 0 us-gaap_FairValueLiabilitiesLevel2ToLevel1TransfersAmount | 0 us-gaap_FairValueLiabilitiesLevel2ToLevel1TransfersAmount
Other assets or liabilities required to be recognized at fair value on a non‑recurring basis | 28,932,000 us-gaap_LiabilitiesFairValueDisclosure | 57,896,000 us-gaap_LiabilitiesFairValueDisclosure |
Nonrecurring Basis |  |  |
Financial Assets and Liabilities Fair Value Measurement |  |  |
Other assets or liabilities required to be recognized at fair value on a non‑recurring basis |  | $ 0 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember | $ 0 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember

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- Definition Represents the number of non-financial assets and liabilities that were impaired during the period. + References No definition available. + Details
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- Definition Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2. + 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 (bb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1. + 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 (bb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Amount of transfers of liabilities measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2. + 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 (bb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Fair value of financial and nonfinancial obligations. + 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=36462937&loc=d3e19207-110258 + Details
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