Loans - TDR - Subsequent Defaults of Completed TDR Modifications (Detail) $ in Millions	3 Months Ended
Mar. 31, 2019 USD ($) Contract	Mar. 31, 2018 USD ($) Contract
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	31,846	32,102
Total Loans \| $	$ 70	$ 117
Credit Card
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	30,733	30,278
Total Loans \| $	$ 57	$ 60
Consumer Banking
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	1,113	1,818
Total Loans \| $	$ 13	$ 22
Consumer Banking \| Auto
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	1,105	1,807
Total Loans \| $	$ 13	$ 21
Consumer Banking \| Home loan
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	0	3
Total Loans \| $	$ 0	$ 1
Consumer Banking \| Retail banking
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	8	8
Total Loans \| $	$ 0	$ 0
Commercial Banking
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	0	6
Total Loans \| $	$ 0	$ 35
Commercial Banking \| Total commercial lending
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	0	6
Total Loans \| $	$ 0	$ 35
Commercial Banking \| Commercial and industrial
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	0	6
Total Loans \| $	$ 0	$ 35
Domestic credit card \| Credit Card
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	14,027	16,339
Total Loans \| $	$ 29	$ 34
International card business \| Credit Card
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	16,706	13,939
Total Loans \| $	$ 28	$ 26

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.

No definition available.

+ Details

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_GeographicDistributionAxis=us-gaap_GeographicDistributionDomesticMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GeographicDistributionAxis=us-gaap_GeographicDistributionForeignMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: