Loans - Commercial Banking: Risk Profile by Internal Risk Rating (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Financing Receivable, Allowance for Credit Loss [Line Items]
Total	$ 312,331	$ 277,340
Commercial Banking
Financing Receivable, Allowance for Credit Loss [Line Items]
Term loans by vintage year one	33,735	19,811
Term loans by vintage year two	11,096	10,398
Term loans by vintage year three	4,458	10,239
Term loans by vintage year four	4,152	5,788
Term loans by vintage year five	2,781	2,919
Term loans by vintage after year five	6,814	8,492
Total term loans	63,036	57,647
Revolving loans	31,594	27,191
Revolving loans converted to term	46	84
Total	94,676	84,922
Commercial Banking \| Commercial and multifamily real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Term loans by vintage year one	10,442	6,838
Term loans by vintage year two	4,484	3,119
Term loans by vintage year three	1,363	3,810
Term loans by vintage year four	2,096	2,738
Term loans by vintage year five	1,601	1,063
Term loans by vintage after year five	4,075	5,282
Total term loans	24,061	22,850
Revolving loans	13,367	12,387
Revolving loans converted to term	25	25
Total	37,453	35,262
Commercial Banking \| Commercial and multifamily real estate \| Noncriticized
Financing Receivable, Allowance for Credit Loss [Line Items]
Term loans by vintage year one	9,527	6,590
Term loans by vintage year two	4,086	2,924
Term loans by vintage year three	1,161	3,393
Term loans by vintage year four	1,671	2,401
Term loans by vintage year five	1,280	793
Term loans by vintage after year five	2,898	3,538
Total term loans	20,623	19,639
Revolving loans	13,254	12,286
Revolving loans converted to term	25	0
Total	33,902	31,925
Commercial Banking \| Commercial and multifamily real estate \| Criticized \| Criticized performing
Financing Receivable, Allowance for Credit Loss [Line Items]
Term loans by vintage year one	814	248
Term loans by vintage year two	376	195
Term loans by vintage year three	202	329
Term loans by vintage year four	412	317
Term loans by vintage year five	302	261
Term loans by vintage after year five	1,061	1,478
Total term loans	3,167	2,828
Revolving loans	113	101
Revolving loans converted to term	0	25
Total	3,280	2,954
Commercial Banking \| Commercial and multifamily real estate \| Criticized \| Criticized nonperforming
Financing Receivable, Allowance for Credit Loss [Line Items]
Term loans by vintage year one	101	0
Term loans by vintage year two	22	0
Term loans by vintage year three	0	88
Term loans by vintage year four	13	20
Term loans by vintage year five	19	9
Term loans by vintage after year five	116	266
Total term loans	271	383
Revolving loans	0	0
Revolving loans converted to term	0	0
Total	271	383
Commercial Banking \| Commercial And Industrial [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Term loans by vintage year one	23,293	12,973
Term loans by vintage year two	6,612	7,279
Term loans by vintage year three	3,095	6,429
Term loans by vintage year four	2,056	3,050
Term loans by vintage year five	1,180	1,856
Term loans by vintage after year five	2,739	3,210
Total term loans	38,975	34,797
Revolving loans	18,227	14,804
Revolving loans converted to term	21	59
Total	57,223	49,660
Commercial Banking \| Commercial And Industrial [Member] \| Noncriticized
Financing Receivable, Allowance for Credit Loss [Line Items]
Term loans by vintage year one	22,105	12,662
Term loans by vintage year two	6,031	7,039
Term loans by vintage year three	2,934	5,506
Term loans by vintage year four	1,809	2,750
Term loans by vintage year five	973	1,730
Term loans by vintage after year five	2,658	3,033
Total term loans	36,510	32,720
Revolving loans	17,187	14,310
Revolving loans converted to term	21	59
Total	53,718	47,089
Commercial Banking \| Commercial And Industrial [Member] \| Criticized \| Criticized performing
Financing Receivable, Allowance for Credit Loss [Line Items]
Term loans by vintage year one	992	279
Term loans by vintage year two	560	188
Term loans by vintage year three	156	838
Term loans by vintage year four	160	207
Term loans by vintage year five	167	120
Term loans by vintage after year five	76	167
Total term loans	2,111	1,799
Revolving loans	964	456
Revolving loans converted to term	0	0
Total	3,075	2,255
Commercial Banking \| Commercial And Industrial [Member] \| Criticized \| Criticized nonperforming
Financing Receivable, Allowance for Credit Loss [Line Items]
Term loans by vintage year one	196	32
Term loans by vintage year two	21	52
Term loans by vintage year three	5	85
Term loans by vintage year four	87	93
Term loans by vintage year five	40	6
Term loans by vintage after year five	5	10
Total term loans	354	278
Revolving loans	76	38
Revolving loans converted to term	0	0
Total	$ 430	$ 316

Financing Receivable, Excluding Accrued Interest, Before Allowance for Credit Loss, Term Loans

No definition available.

Name:	cof_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLossTermLoans
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

Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919230-210447

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 3B
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL120267963-210447

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable originated more than five years prior to current fiscal year. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 3B
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL120267963-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 6
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919232-210447

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI https://asc.fasb.org/extlink&oid=124267575&loc=SL82922352-210448

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable that can be withdrawn, repaid, and redrawn.

+ 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 https://asc.fasb.org/extlink&oid=124255953&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 https://asc.fasb.org/extlink&oid=124267575&loc=SL82922352-210448

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 3B
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL120267963-210447

+ Details

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

- Definition

Amortized cost excluding accrued interest, of revolving financing receivable converted to term loan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 3B
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL120267963-210447

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI https://asc.fasb.org/extlink&oid=124267575&loc=SL82922352-210448

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 6A
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL120270059-210447

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable originated four years prior to current fiscal year. Excludes net investment in lease.

+ 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 https://asc.fasb.org/extlink&oid=124267575&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 6
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919232-210447

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 3B
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL120267963-210447

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable originated three years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable originated in current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable originated two years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost excluding accrued interest of financing receivable originated in fiscal year prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYear
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: