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Employee Benefit Plans (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2012 Qualified pension plan | Dec. 31, 2011 Non-qualified pension plans | Dec. 31, 2012 Postretirement Benefits Other than Pensions
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Cash flows |  |  |
Employer's contribution in current year | $ 11.3 | $ 5.6 |
Employer's expected contribution in following year |  |  | $ 1.3

X
- Definition The contributions paid to the plan during the current year. Contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions. + References No definition available. + Details
Name: | cog_DefinedBenefitPlanEmployerContributionsInCurrentYear
Namespace Prefix: | cog_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The expected contributions paid in the following year. Contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions. + References No definition available. + Details
Name: | cog_DefinedBenefitPlanEmployerContributionsInFollowingYear
Namespace Prefix: | cog_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_DefinedBenefitPlanEstimatedFutureEmployerContributionsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration