Loans - TDR - Subsequent Defaults of Completed TDR Modifications (Detail) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($) Contract	Dec. 31, 2016 USD ($) Contract	Dec. 31, 2015 USD ($) Contract
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	116,523	91,597	85,393
Total Loans \| $	$ 594	$ 550	$ 274
Credit Card Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	106,762	82,748	76,696
Total Loans \| $	$ 204	$ 155	$ 152
Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	9,515	8,691	8,687
Total Loans \| $	$ 120	$ 112	$ 103
Consumer Portfolio Segment [Member] \| Auto
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	9,446	8,587	8,647
Total Loans \| $	$ 109	$ 96	$ 99
Consumer Portfolio Segment [Member] \| Home loan
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	28	56	14
Total Loans \| $	$ 7	$ 7	$ 2
Consumer Portfolio Segment [Member] \| Retail banking
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	41	48	26
Total Loans \| $	$ 4	$ 9	$ 2
Commercial Banking
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	246	158	10
Total Loans \| $	$ 270	$ 283	$ 19
Commercial Banking \| Total commercial lending
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	244	151	7
Total Loans \| $	$ 269	$ 282	$ 19
Commercial Banking \| Commercial and multifamily real estate
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	0	1	0
Total Loans \| $	$ 0	$ 1	$ 0
Commercial Banking \| Commercial and industrial
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	244	150	7
Total Loans \| $	$ 269	$ 281	$ 19
Commercial Banking \| Small-ticket commercial real estate
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	2	7	3
Total Loans \| $	$ 1	$ 1	$ 0
Geographic Distribution, Domestic [Member] \| Credit Card Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	55,121	42,250	42,808
Total Loans \| $	$ 111	$ 73	$ 71
Geographic Distribution, Foreign [Member] \| Credit Card Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts \| Contract	51,641	40,498	33,888
Total Loans \| $	$ 93	$ 82	$ 81

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://www.xbrl.org/2003/role/presentationRef
-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_CommercialAndMultifamilyRealEstateMember
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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=cof_SmallTicketRealEstateMember
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: