Derivative Credit Risk Valuation Adjustment, Derivative Assets (Liabilities)

No definition available.

Name:	cof_DerivativeCreditRiskValuationAdjustmentDerivativeAssetsLiabilities
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Equity Securities Classified as Other Assets, Fair Value Disclosure

+ References

No definition available.

+ Details

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

- Definition

Other Assets, Fair Value Disclosure [Abstract]

+ References

No definition available.

+ Details

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

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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

- References

No definition available.

+ Details

Name:	us-gaap_AssetsFairValueDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

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

- Definition

Amount of unrealized gain (loss) on investment in debt and equity securities.

+ References

No definition available.

+ Details

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

- Definition

Carrying amount as of the balance sheet date of assets held under deferred compensation agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and elected not to be offset.

+ References

+ Details

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

- Definition

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, and obligation to return cash collateral under master netting arrangements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226003-175313

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=117331979&loc=d3e41228-113958

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

Name:	us-gaap_DerivativeAssetFairValueGrossLiabilityAndObligationToReturnCashOffset
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

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

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

+ Details

Name:	us-gaap_DerivativeLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value of asset associated with financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, and right to receive cash collateral under master netting arrangements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226003-175313

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=117331979&loc=d3e41228-113958

+ Details

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

- Definition

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and elected not to be offset.

+ References

+ Details

Name:	us-gaap_DerivativeLiabilityFairValueGrossLiabilityIncludingNotSubjectToMasterNettingArrangement
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_FinancialLiabilitiesFairValueDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

+ References

No definition available.

+ Details

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

- Definition

Fair value portion of other assets.

+ References

No definition available.

+ Details

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

- Definition

Fair value portion of interest continued to be held by a transferor after transferring financial assets to a third party.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=cof_FairValueInputsLevel1AndLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
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=us-gaap_CommercialMortgageBackedSecuritiesMember
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:

Fair Value Measurement - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	$ 99,165	$ 100,445
Loans held for sale	908	596
Derivative assets, gross amount	2,869	3,415
Derivative asset, netting adjustments	(844)	(1,148)
Derivative asset	2,025	2,267
Liabilities:
Derivative liabilities, gross amount	1,651	1,518
Derivative liability, netting adjustments	(679)	(739)
Derivative liability	972	779
Net valuation allowance	26	31
Level 1
Assets:
Loans held for sale	0	0
Level 2
Assets:
Loans held for sale	2,082	2,214
Level 3
Assets:
Loans held for sale	0	0
Recurring
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	99,165	100,445
Loans held for sale	908	596
Derivative asset	2,025	2,267
Other assets	676	1,037
Total assets	102,774	104,345
Liabilities:
Derivative liability	972	779
Recurring \| Level 1
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	9,447	9,460
Loans held for sale	0	0
Derivative assets, gross amount	463	268
Other assets	624	430
Total assets	10,534	10,158
Other assets:
Deferred compensation plan assets	449	414
Liabilities:
Derivative liabilities, gross amount	528	271
Recurring \| Level 2
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	89,382	90,546
Loans held for sale	908	596
Derivative assets, gross amount	2,249	3,006
Other assets	2	552
Total assets	92,541	94,700
Liabilities:
Derivative liabilities, gross amount	1,000	1,137
Recurring \| Level 3
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	336	439
Loans held for sale	0	0
Derivative assets, gross amount	157	141	$ 147
Other assets	50	55
Total assets	543	635
Other assets:
Retained interests in securitizations	50	55
Liabilities:
Derivative liabilities, gross amount	123	110	$ 81
Recurring \| Fair Value Inputs Level 1 And Level 2 [Member]
Other assets:
Equity securities	177	568
Liabilities:
Debt and Equity Securities, Unrealized Gain (Loss)	(38)	535
U.S. Treasury securities
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	9,248	9,318
U.S. Treasury securities \| Recurring
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	9,248	9,318
U.S. Treasury securities \| Recurring \| Level 1
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	9,248	9,318
U.S. Treasury securities \| Recurring \| Level 2
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	0	0
U.S. Treasury securities \| Recurring \| Level 3
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	0	0
RMBS
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	75,622	76,703
RMBS \| Recurring
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	75,622	76,703
RMBS \| Recurring \| Level 1
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	0	0
RMBS \| Recurring \| Level 2
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	75,296	76,375
RMBS \| Recurring \| Level 3
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	326	328
CMBS \| Recurring
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	11,274	11,735
CMBS \| Recurring \| Level 1
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	0	0
CMBS \| Recurring \| Level 2
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	11,264	11,624
CMBS \| Recurring \| Level 3
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	10	111
Other securities
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	3,021	2,689
Other securities \| Recurring
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	3,021	2,689
Other securities \| Recurring \| Level 1
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	199	142
Other securities \| Recurring \| Level 2
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	2,822	2,547
Other securities \| Recurring \| Level 3
Assets:
Securities available for sale (amortized cost of $97.8 billion and $97.6 billion and allowance for credit losses of $2 million and $1 million as of March 31, 2021 and December 31, 2020, respectively)	$ 0	$ 0