Investment Securities - Contractual Maturities and Weighted-Average Yields of Securities (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2024	Dec. 31, 2023
Fair value of securities available for sale:
Due in 1 Year or Less	$ 2,761
Due > 1 Year through 5 Years	7,798
Due > 5 Years through 10 Years	4,617
Due > 10 Years	63,222
Total	78,398	$ 79,117
Amortized cost of securities available for sale
Due in 1 Year or Less	2,788
Due > 1 Year through 5 Years	7,986
Due > 5 Years through 10 Years	5,013
Due > 10 Years	72,672
Total	$ 88,459	88,063
Weighted-average yield for securities available for sale
Due in 1 Year or Less	3.50%
Due > 1 Year through 5 Years	4.43%
Due > 5 Years through 10 Years	3.52%
Due > 10 Years	3.10%
Total	3.26%
U.S. Treasury securities
Fair value of securities available for sale:
Due in 1 Year or Less	$ 1,892
Due > 1 Year through 5 Years	3,542
Due > 5 Years through 10 Years	294
Due > 10 Years	0
Total	5,728	5,282
Amortized cost of securities available for sale
Total	5,764	5,330
RMBS, Agency
Fair value of securities available for sale:
Due in 1 Year or Less	2
Due > 1 Year through 5 Years	92
Due > 5 Years through 10 Years	1,073
Due > 10 Years	60,554
Total	61,721	62,948
Amortized cost of securities available for sale
Total	71,153	71,294
RMBS, Non-agency
Fair value of securities available for sale:
Due in 1 Year or Less	0
Due > 1 Year through 5 Years	0
Due > 5 Years through 10 Years	5
Due > 10 Years	672
Total	677	690
Amortized cost of securities available for sale
Total	600	610
Total RMBS
Fair value of securities available for sale:
Due in 1 Year or Less	2
Due > 1 Year through 5 Years	92
Due > 5 Years through 10 Years	1,078
Due > 10 Years	61,226
Total	62,398	63,638
Amortized cost of securities available for sale
Total	$ 71,753	71,904
Weighted-average yield for securities available for sale
Weighted-average expected life	7 years 9 months 18 days
Agency CMBS
Fair value of securities available for sale:
Due in 1 Year or Less	$ 494
Due > 1 Year through 5 Years	2,482
Due > 5 Years through 10 Years	3,221
Due > 10 Years	1,996
Total	8,193	8,323
Amortized cost of securities available for sale
Total	$ 8,867	8,961
Weighted-average yield for securities available for sale
Weighted-average expected life	4 years 4 months 24 days
Other securities
Fair value of securities available for sale:
Due in 1 Year or Less	$ 373
Due > 1 Year through 5 Years	1,682
Due > 5 Years through 10 Years	24
Due > 10 Years	0
Total	2,079	1,874
Amortized cost of securities available for sale
Total	$ 2,075	$ 1,868

Available-for-sale Securities, Expected Debt Maturities, Amortized Cost Basis, Rolling Maturity

No definition available.

Name:	cof_AvailableforsaleSecuritiesExpectedDebtMaturitiesAmortizedCostBasisRollingMaturity
Namespace Prefix:	cof_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Available-for-sale Securities, Expected Debt Maturities, Fair Value, Rolling Maturity [Abstract]

+ References

No definition available.

+ Details

Name:	cof_AvailableforsaleSecuritiesExpectedDebtMaturitiesFairValueRollingMaturityAbstract
Namespace Prefix:	cof_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Available-for-sale Securities, Weighted Average Yield [Abstract]

+ References

No definition available.

+ Details

Name:	cof_AvailableforsaleSecuritiesWeightedAverageYieldAbstract
Namespace Prefix:	cof_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Maturities, Available-for-Sale, Weighted Average Expected Life

+ References

No definition available.

+ Details

Name:	cof_DebtMaturitiesAvailableforSaleWeightedAverageExpectedLife
Namespace Prefix:	cof_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next rolling fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-3

+ Details

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

- Definition

Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next rolling fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

+ Details

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

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing after rolling tenth fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-3

+ Details

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

- Definition

Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing after rolling tenth fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

+ Details

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

- Definition

Amortized cost of investment in debt security, measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in rolling sixth through tenth fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-3

+ Details

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

- Definition

Fair value of investment in debt security, measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in rolling sixth through tenth fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

+ Details

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

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next rolling fiscal year through fifth year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-3

+ Details

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

- Definition

Fair value of investment in debt security, measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next rolling fiscal year through fifth year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

+ Details

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

- Definition

Amortized cost excluding accrued interest, before allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479130/326-30-45-1

+ Details

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

- Definition

Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479130/326-30-45-1

+ Details

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

- Definition

Weighted average yield of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), maturing after tenth rolling fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Regulation S-K (SK)
-Number 229
-Section 1403
-Paragraph (a)
-Subparagraph (4)
-Publisher SEC

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Industry Guide
-Number 3
-Section II
-Paragraph B
-Publisher SEC

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleMaturityRollingAfter10YearsWeightedAverageYield
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted average yield of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), maturing in sixth through tenth rolling fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Regulation S-K (SK)
-Number 229
-Section 1403
-Paragraph (a)
-Subparagraph (3)
-Publisher SEC

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Industry Guide
-Number 3
-Section II
-Paragraph B
-Publisher SEC

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleMaturityRollingAfterFiveThroughTenYearsWeightedAverageYield
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted average yield of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), maturing in second through fifth rolling fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Regulation S-K (SK)
-Number 229
-Section 1403
-Paragraph (a)
-Subparagraph (2)
-Publisher SEC

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Industry Guide
-Number 3
-Section II
-Paragraph B
-Publisher SEC

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleMaturityRollingAfterOneThroughFiveYearsWeightedAverageYield
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted average yield of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), maturing in next rolling fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Regulation S-K (SK)
-Number 229
-Section 1403
-Paragraph (a)
-Subparagraph (1)
-Publisher SEC

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Industry Guide
-Number 3
-Section II
-Paragraph B
-Publisher SEC

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleMaturityRollingWithinOneYearWeightedAverageYield
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted average yield of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Regulation S-K (SK)
-Number 229
-Section 1403
-Paragraph (a)
-Publisher SEC

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleWeightedAverageYield
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=cof_ResidentialMortgageBackedSecuritiesAgencyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=cof_ResidentialMortgageBackedSecuritiesNonagencyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=cof_CommercialMortgageBackedSecuritiesAgencyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_OtherDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: