Loans - Commercial Banking: Risk Profile by Internal Risk Rating (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans held for investment	$ 249,597	$ 251,624	$ 251,512
Commercial Banking
Financing Receivable, Allowance for Credit Loss [Line Items]
Term Loans by Vintage Year One	6,831	14,262
Term Loans by Vintage Year Two	12,119	14,181
Term Loans by Vintage Year Three	12,396	8,366
Term Loans by Vintage Year Four	7,278	4,818
Term Loans by Vintage Year Five	3,842	3,990
Term Loans by Vintage After Year Five	10,773	9,561
Total Term Loans	53,239	55,178
Revolving Loans	20,542	20,455
Revolving Loans Converted to Term	40	147
Total loans held for investment	73,821	75,780
Loans under Paycheck Protection Program	296	238
Commercial Banking \| Commercial and multifamily real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Term Loans by Vintage Year One	1,792	4,111
Term Loans by Vintage Year Two	3,728	5,389
Term Loans by Vintage Year Three	4,765	3,777
Term Loans by Vintage Year Four	3,366	1,798
Term Loans by Vintage Year Five	1,345	2,043
Term Loans by Vintage After Year Five	6,551	6,312
Total Term Loans	21,547	23,430
Revolving Loans	8,044	7,226
Revolving Loans Converted to Term	25	25
Total loans held for investment	29,616	30,681
Commercial Banking \| Commercial and multifamily real estate \| Noncriticized
Financing Receivable, Allowance for Credit Loss [Line Items]
Term Loans by Vintage Year One	1,636	3,791
Term Loans by Vintage Year Two	3,437	4,932
Term Loans by Vintage Year Three	4,331	3,232
Term Loans by Vintage Year Four	2,956	1,437
Term Loans by Vintage Year Five	1,025	1,649
Term Loans by Vintage After Year Five	4,838	4,904
Total Term Loans	18,223	19,945
Revolving Loans	7,983	7,114
Revolving Loans Converted to Term	0	0
Total loans held for investment	26,206	27,059
Commercial Banking \| Commercial and multifamily real estate \| Criticized \| Criticized performing
Financing Receivable, Allowance for Credit Loss [Line Items]
Term Loans by Vintage Year One	156	320
Term Loans by Vintage Year Two	291	446
Term Loans by Vintage Year Three	426	515
Term Loans by Vintage Year Four	381	355
Term Loans by Vintage Year Five	310	391
Term Loans by Vintage After Year Five	1,459	1,258
Total Term Loans	3,023	3,285
Revolving Loans	61	112
Revolving Loans Converted to Term	25	25
Total loans held for investment	3,109	3,422
Commercial Banking \| Commercial and multifamily real estate \| Criticized \| Criticized nonperforming
Financing Receivable, Allowance for Credit Loss [Line Items]
Term Loans by Vintage Year One	0	0
Term Loans by Vintage Year Two	0	11
Term Loans by Vintage Year Three	8	30
Term Loans by Vintage Year Four	29	6
Term Loans by Vintage Year Five	10	3
Term Loans by Vintage After Year Five	254	150
Total Term Loans	301	200
Revolving Loans	0	0
Revolving Loans Converted to Term	0	0
Total loans held for investment	301	200
Commercial Banking \| Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Term Loans by Vintage Year One	5,039	10,151
Term Loans by Vintage Year Two	8,391	8,792
Term Loans by Vintage Year Three	7,631	4,589
Term Loans by Vintage Year Four	3,912	3,020
Term Loans by Vintage Year Five	2,497	1,947
Term Loans by Vintage After Year Five	4,222	3,249
Total Term Loans	31,692	31,748
Revolving Loans	12,498	13,229
Revolving Loans Converted to Term	15	122
Total loans held for investment	44,205	45,099
Commercial Banking \| Commercial and industrial \| Noncriticized
Financing Receivable, Allowance for Credit Loss [Line Items]
Term Loans by Vintage Year One	4,905	9,761
Term Loans by Vintage Year Two	8,160	7,890
Term Loans by Vintage Year Three	6,730	4,043
Term Loans by Vintage Year Four	3,563	2,717
Term Loans by Vintage Year Five	2,305	1,832
Term Loans by Vintage After Year Five	4,001	3,034
Total Term Loans	29,664	29,277
Revolving Loans	11,596	11,548
Revolving Loans Converted to Term	15	80
Total loans held for investment	41,275	40,905
Commercial Banking \| Commercial and industrial \| Criticized \| Criticized performing
Financing Receivable, Allowance for Credit Loss [Line Items]
Term Loans by Vintage Year One	87	316
Term Loans by Vintage Year Two	169	794
Term Loans by Vintage Year Three	844	521
Term Loans by Vintage Year Four	275	252
Term Loans by Vintage Year Five	163	106
Term Loans by Vintage After Year Five	200	215
Total Term Loans	1,738	2,204
Revolving Loans	734	1,498
Revolving Loans Converted to Term	0	42
Total loans held for investment	2,472	3,744
Commercial Banking \| Commercial and industrial \| Criticized \| Criticized nonperforming
Financing Receivable, Allowance for Credit Loss [Line Items]
Term Loans by Vintage Year One	47	74
Term Loans by Vintage Year Two	62	108
Term Loans by Vintage Year Three	57	25
Term Loans by Vintage Year Four	74	51
Term Loans by Vintage Year Five	29	9
Term Loans by Vintage After Year Five	21	0
Total Term Loans	290	267
Revolving Loans	168	183
Revolving Loans Converted to Term	0	0
Total loans held for investment	$ 458	$ 450

Financing receivable past due and not past due under paycheck protection program

No definition available.

Name:	cof_FinancingReceivablePastDueAndNotPastDueUnderPaycheckProtectionProgram
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financing Receivable, Total Term Loans, Before Allowance For Credit Loss

+ References

No definition available.

+ Details

Name:	cof_FinancingReceivableTotalTermLoansBeforeAllowanceForCreditLoss
Namespace Prefix:	cof_
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_FinancingReceivableAllowanceForCreditLossesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of financing receivable originated more than five years prior to current fiscal year.

+ References

+ Details

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

- Definition

Amount of financing receivable originated four years prior to current fiscal year.

+ References

+ Details

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

- Definition

Amount of financing receivable originated in current fiscal year.

+ References

+ Details

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

- Definition

Amount of financing receivable originated in fiscal year prior to current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919232-210447

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922352-210448

+ Details

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

- Definition

Amount of financing receivable originated three years prior to current fiscal year.

+ References

+ Details

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

- Definition

Amount of financing receivable originated two years prior to current fiscal year.

+ References

+ Details

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

- Definition

Amount of financing receivable that can be withdrawn, repaid and redrawn.

+ References

+ Details

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

- Definition

Amount of revolving financing receivable converted to term loan.

+ References

+ Details

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

- Definition

Amount, before allowance for credit loss, of financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922352-210448

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919230-210447

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

- 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_InternalCreditAssessmentAxis=us-gaap_PassMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_CriticizedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentPerformanceStatusAxis=us-gaap_PerformingFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentPerformanceStatusAxis=us-gaap_NonperformingFinancingReceivableMember
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: