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Summary of Significant Accounting Policies - Arrangements (Details) - USD ($) $ in Billions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Collaborative Arrangement |  |  |
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] |  |  |
Reduction of interest income | $ 1.1 | $ 1.0 | $ 1.3

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- Definition Collaborative Arrangement, Revenue As Reduction To Interest Income + References No definition available. + Details
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Namespace Prefix: | cof_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- 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

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- Details
Name: | us-gaap_TypeOfArrangementAxis=us-gaap_CollaborativeArrangementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |