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Employee Benefit Plans - Defined Contribution Plan Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) age	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, cost | $	$ 350	$ 316	$ 291
Associate Savings Plan
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, age eligible to participate | age	18

X

- Definition

Defined Contribution Plan, Age Eligible to Participate

+ References

No definition available.

+ Details

Name:	cof_DefinedContributionPlanAgeEligibletoParticipate
Namespace Prefix:	cof_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Amount of cost for defined contribution plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ Details

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

X

- 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

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

X

- Details

Name:	us-gaap_RetirementPlanNameAxis=cof_AssociateSavingsPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: