Summary of Significant Accounting Policies - Loans and Allowance for Loan and Lease Losses (Details)	12 Months Ended
Dec. 31, 2018
Loans and Leases Receivable Disclosure [Line Items]
Threshold period past due for delinquency status of financing receivables	30 days
Credit Card Portfolio Segment
Loans and Leases Receivable Disclosure [Line Items]
Loan origination fees and direct loan origination costs amortization period	12 months
Threshold period past due for write-off of financing receivable	180 days
Credit Card Portfolio Segment \| Privileges Revoked
Loans and Leases Receivable Disclosure [Line Items]
Threshold period past due for write-off of financing receivable	120 days
Credit Card Portfolio Segment \| Notification of Death
Loans and Leases Receivable Disclosure [Line Items]
Threshold period past due for write-off of financing receivable	60 days
Credit Card Portfolio Segment \| Chapter Seven Bankruptcy
Loans and Leases Receivable Disclosure [Line Items]
Threshold period past due for write-off of financing receivable	30 days
Consumer Portfolio Segment
Loans and Leases Receivable Disclosure [Line Items]
Threshold period past due for nonperforming status of financing receivables	90 days
Consumer Portfolio Segment \| Notification of Death
Loans and Leases Receivable Disclosure [Line Items]
Threshold period past due for nonperforming status of financing receivables	60 days
Consumer Portfolio Segment \| Auto
Loans and Leases Receivable Disclosure [Line Items]
Threshold period past due for nonperforming status of financing receivables	120 days
Consumer Portfolio Segment \| Home loan
Loans and Leases Receivable Disclosure [Line Items]
Threshold period past due for nonperforming status of financing receivables	180 days
Consumer Portfolio Segment \| Small Business Banking Loans \| Maximum
Loans and Leases Receivable Disclosure [Line Items]
Threshold period past due for nonperforming status of financing receivables	120 days
Consumer Portfolio Segment \| Other Consumer Loans
Loans and Leases Receivable Disclosure [Line Items]
Threshold Period Following Bankruptcy Notice for Write-off of Financing Receivable	40 days
Consumer Portfolio Segment \| Auto and home loan [Member] \| Chapter Seven Bankruptcy
Loans and Leases Receivable Disclosure [Line Items]
Threshold period past due for nonperforming status of financing receivables	60 days

Loan Commitment and Origination Fees and Discounts or Premiums, Amortization Period

No definition available.

Name:	cof_LoanCommitmentandOriginationFeesandDiscountsorPremiumsAmortizationPeriod
Namespace Prefix:	cof_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Threshold Period Following Bankruptcy Notice for Write-off of Financing Receivable

+ References

No definition available.

+ Details

Name:	cof_ThresholdPeriodFollowingBankruptcyNoticeforWriteoffofFinancingReceivable
Namespace Prefix:	cof_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Threshold Period Past Due for Delinquency Status of Financing Receivables

+ References

No definition available.

+ Details

Name:	cof_ThresholdPeriodPastDueforDelinquencyStatusofFinancingReceivables
Namespace Prefix:	cof_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Threshold Period Past Due for Nonperforming Status of Financing Receivables

+ References

No definition available.

+ Details

Name:	cof_ThresholdPeriodPastDueforNonperformingStatusofFinancingReceivables
Namespace Prefix:	cof_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- 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_LoansAndLeasesReceivableDisclosureLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Threshold period past due for financing receivable to write off as uncollectible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_ThresholdPeriodPastDueForWriteoffOfFinancingReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=cof_PrivilegesRevokedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=cof_NotificationofDeathMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=cof_ChapterSevenBankruptcyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_AutomobileLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cof_SmallBusinessBankingLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cof_OtherConsumerLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cof_AutoandhomeloanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: