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Employee Benefit Plans - Defined Contribution Plan (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Defined Contribution Plan Disclosure [Line Items]      
Defined contribution plan, cost $ 214us-gaap_DefinedContributionPlanCostRecognized $ 206us-gaap_DefinedContributionPlanCostRecognized $ 167us-gaap_DefinedContributionPlanCostRecognized
Associate Savings Plan      
Defined Contribution Plan Disclosure [Line Items]      
Defined contribution plan, age eligible to participate 18cof_DefinedContributionPlanAgeEligibletoParticipate
/ us-gaap_DefinedContributionPlanNameAxis
= cof_AssociateSavingsPlanMember