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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

X

- Definition

Collaborative Arrangement, Revenue As Reduction To Interest Income

+ References

No definition available.

+ Details

Name:	cof_CollaborativeArrangementRevenueAsReductionToInterestIncome
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

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