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Commitments, Contingencies, Guarantees, and Others - Unfunded Lending Commitments: Contractual Amount and Carrying Value (Details) - USD ($) $ in Millions | Dec. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Loss Contingencies [Line Items] |  |
Unfunded lending commitments, contractual amount | $ 414,499 | $ 386,732
Off-balance sheet lending commitment, carrying value | 149 | 176
Credit Card |  |
Loss Contingencies [Line Items] |  |
Unused commitments to extend credit, contractual amount | 368,508 | 349,594
Other excluding credit card |  |
Loss Contingencies [Line Items] |  |
Unused commitments to extend credit, contractual amount | 44,572 | 35,836
Off-balance sheet lending commitment, carrying value | 125 | 144
Advised line of credit | 3,900 | 1,800
Letter of credit |  |
Loss Contingencies [Line Items] |  |
Standby letters of credit and commercial letters of credit, contractual amount | 1,419 | 1,302
Off-balance sheet lending commitment, carrying value | $ 24 | $ 32

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- Definition The contractual amount of line of credit that drawings are subject to our satisfactory evaluation of the customer and are cancelable upon proper notice. + References No definition available. + Details
Name: | cof_AdvisedLineofCredit
Namespace Prefix: | cof_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Letter of credit issued contractual amount and unused commitment to extend credit + References No definition available. + Details
Name: | cof_LetterofCreditIssuedContractualAmountandUnusedCommitmenttoExtendCredit
Namespace Prefix: | cof_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The carrying value of off-balance sheet lending-related financial instruments (e.g., commitments and guarantees), which represents the allowance for lending-related commitments, deferred revenue and the guarantee liability. + References No definition available. + Details
Name: | cof_OffBalanceSheetLendingCommitmentCarryingValue
Namespace Prefix: | cof_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

X
- Definition Unused amount of commitments to extend credit to customers. + References No definition available. + Details
Name: | us-gaap_UnusedCommitmentsToExtendCredit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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

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- Details
Name: | us-gaap_CreditFacilityAxis=us-gaap_LetterOfCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |