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v3.20.1

Summary of Significant Accounting Policies - Revenue Recognition (Details) - Credit Card | 3 Months Ended
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Mar. 31, 2020
Loans and Leases Receivable Disclosure [Line Items] |
Loan Commitment and Origination Fees and Discounts or Premiums, Amortization Period | 12 months
Amortization term of annual membership fees | 12 months

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- Definition Amortization term of credit card annual membership fees + References No definition available. + Details
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Namespace Prefix: | cof_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Loan Commitment and Origination Fees and Discounts or Premiums, Amortization Period + References No definition available. + Details
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Namespace Prefix: | cof_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=cof_CreditCardPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |