Derivative Instruments and Hedging Activities - Hedged Items in Fair Value Hedging Relationship (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Hedged Items In Fair Value Hedging Relationship [Line Items]
Hedged Assets, Fair Value Hedge, Cumulative Increase (Decrease)	$ (1)
Hedged Assets, Discontinued Fair Value Hedge, Cumulative Increase (Decrease)	249
Prepayable Financial Asset Closed Portfolio, Last-of-Layer, Amortized Cost	225
Available-for-sale securities
Hedged Items In Fair Value Hedging Relationship [Line Items]
Carrying Amount of Assets	9,948	$ 9,797
Hedged Assets, Fair Value Hedge, Cumulative Increase (Decrease)	485	590
Hedged Assets, Discontinued Fair Value Hedge, Cumulative Increase (Decrease)	194	200
Interest-bearing deposits
Hedged Items In Fair Value Hedging Relationship [Line Items]
Carrying Amount Of Liabilities	(10,820)	(11,312)
Hedged Liabilities, Fair Value Hedge, Cumulative Increase (Decrease)	(155)	(213)
Hedged Liabilities, Discontinued Fair Value Hedge, Cumulative Increase (Decrease)	0	0
Securitized debt obligations
Hedged Items In Fair Value Hedging Relationship [Line Items]
Carrying Amount Of Liabilities	(7,536)	(7,609)
Hedged Liabilities, Fair Value Hedge, Cumulative Increase (Decrease)	(96)	(171)
Hedged Liabilities, Discontinued Fair Value Hedge, Cumulative Increase (Decrease)	15	20
Senior and subordinated notes
Hedged Items In Fair Value Hedging Relationship [Line Items]
Carrying Amount Of Liabilities	(20,265)	(21,927)
Hedged Liabilities, Fair Value Hedge, Cumulative Increase (Decrease)	(913)	(1,282)
Hedged Liabilities, Discontinued Fair Value Hedge, Cumulative Increase (Decrease)	$ (633)	$ (666)

Carry amount of assets hedged in fair value hedging relationship

No definition available.

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

- Definition

The carry amount of liabilities in fair value hedging relationship

+ References

No definition available.

+ Details

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

- Definition

[Line Items] for Table shows the carrying amount and associated cumulative basis adjustment related to the application of hedge accounting that is included in the carrying amount of hedged assets and liabilities in fair value hedging relationships.

+ References

No definition available.

+ Details

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

- Definition

Amount of cumulative increase (decrease) in fair value of hedged asset in fair value hedge, attributable to hedged risk, remaining after discontinued hedge.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL109999712-113959

+ Details

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

- Definition

Amount of cumulative increase (decrease) in fair value of hedged asset in fair value hedge, attributable to hedged risk.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL109999712-113959

+ Details

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

- Definition

Amount of cumulative increase (decrease) in fair value of hedged liability in fair value hedge, attributable to hedged risk, remaining after discontinued hedge.

+ References

+ Details

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

- Definition

Amount of cumulative increase (decrease) in fair value of hedged liability in fair value hedge, attributable to hedged risk.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL109999712-113959

+ Details

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

- Definition

Amortized cost of closed portfolio of prepayable financial asset, used to designate hedging relationship in which hedged item is last layer expected to be remaining at end of hedging relationship.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EEE
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL109999718-113959

+ Details

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

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_AvailableforsaleSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_InterestBearingDepositsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=cof_SecuritizedDebtObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=cof_SeniorAndSubordinatedNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: