Loans - 90+ Day Delinquent Loans Accruing Interest and Nonperforming Loans (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
90 Days Past Due and Accruing	$ 1,084	$ 1,084	$ 2,407
Nonperforming Loans	1,049	1,049	$ 983
Nonperforming Loans Without an Allowance	$ 423	$ 423
Percentage, 90 Days Past Due and Accruing	0.40%	0.40%	0.90%
Percentage, Nonperforming Loans	0.40%	0.40%	0.40%
Percentage, Nonperforming Loans Without an Allowance	0.20%	0.20%
Interest income for loans classified as nonperforming	$ 4	$ 22
Credit Card
Financing Receivable, Past Due [Line Items]
90 Days Past Due and Accruing	1,083	1,083	$ 2,407
Nonperforming Loans	21	21	25
Nonperforming Loans Without an Allowance	0	0
Credit Card \| Domestic credit card:
Financing Receivable, Past Due [Line Items]
90 Days Past Due and Accruing	1,020	1,020	2,277
Nonperforming Loans Without an Allowance	0	0
Credit Card \| International card businesses:
Financing Receivable, Past Due [Line Items]
90 Days Past Due and Accruing	63	63	130
Nonperforming Loans	21	21	25
Nonperforming Loans Without an Allowance	0	0
Consumer Banking
Financing Receivable, Past Due [Line Items]
90 Days Past Due and Accruing	1	1	0
Nonperforming Loans	260	260	510
Nonperforming Loans Without an Allowance	0	0
Consumer Banking \| Auto
Financing Receivable, Past Due [Line Items]
90 Days Past Due and Accruing	0	0	0
Nonperforming Loans	235	235	487
Nonperforming Loans Without an Allowance	0	0
Consumer Banking \| Retail banking
Financing Receivable, Past Due [Line Items]
90 Days Past Due and Accruing	1	1	0
Nonperforming Loans	25	25	23
Nonperforming Loans Without an Allowance	0	0
Commercial Banking
Financing Receivable, Past Due [Line Items]
90 Days Past Due and Accruing	0	0	0
Nonperforming Loans	768	768	448
Nonperforming Loans Without an Allowance	423	423
Commercial Banking \| Commercial and multifamily real estate
Financing Receivable, Past Due [Line Items]
90 Days Past Due and Accruing	0	0	0
Nonperforming Loans	182	182	38
Nonperforming Loans Without an Allowance	180	180
Commercial Banking \| Commercial and industrial
Financing Receivable, Past Due [Line Items]
90 Days Past Due and Accruing	0	0	0
Nonperforming Loans	586	586	$ 410
Nonperforming Loans Without an Allowance	$ 243	$ 243

Financing Receivable, Nonaccrual, No Allowance, Percent Past Due

No definition available.

Name:	cof_FinancingReceivableNonaccrualNoAllowancePercentPastDue
Namespace Prefix:	cof_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Financing Receivable, Percent 90 Days Past Due and Still Accruing

+ References

No definition available.

+ Details

Name:	cof_FinancingReceivablePercent90DaysPastDueandStillAccruing
Namespace Prefix:	cof_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of interest income on financing receivable on nonaccrual status.

+ References

+ Details

Name:	us-gaap_FinancingReceivableNonaccrualInterestIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of financing receivable on nonaccrual status with no allowance for credit loss.

+ References

+ Details

Name:	us-gaap_FinancingReceivableNonaccrualNoAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage of nonaccrual financing receivable balance outstanding that is past due.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableNonaccrualPercentPastDue1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of financing receivable 90 days or more past due and still accruing.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 16
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919258-210447

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5111-111524

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 80
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922355-210448

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of financing receivable on nonaccrual status.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5111-111524

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 16
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919258-210447

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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_FinancingReceivableRecordedInvestmentPastDueLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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_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:

- 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=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_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: