XML 120 R102.htm IDEA: XBRL DOCUMENT
v3.20.4

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
Name: | us-gaap_DefinedContributionPlanCostRecognized
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
Name: | us-gaap_DefinedContributionPlanDisclosureLineItems
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: |