Loans - Credit Card: Risk Profile by Geographic Region and Delinquency Status (Detail) (USD $) In Millions, unless otherwise specified	0 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	$ 208,316	$ 197,199	$ 205,889
Amount, 30 plus day delinquencies	6,070	5,833
Amount, 90 plus day delinquencies	1,805	1,872
30 Plus day delinquencies (as percent)	2.91%cof_FinancingReceivablePercentPastDue	2.96%cof_FinancingReceivablePercentPastDue
90 plus day delinquencies (as percent)	0.87%	0.95%
Credit Card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	85,876	81,305
Percentage of portfolio	100.00%	100.00%
Amount, 30 plus day delinquencies	2,832	2,881
Amount, 90 plus day delinquencies	1,292	1,328
30 Plus day delinquencies (as percent)	3.30%	3.54%
90 plus day delinquencies (as percent)	1.50%	1.63%
Credit Card \| California
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	8,574	7,940
Percentage of portfolio	10.00%	9.80%
Credit Card \| New York
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	5,610	5,277
Percentage of portfolio	6.50%	6.50%
Credit Card \| Texas
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	5,382	4,993
Percentage of portfolio	6.30%	6.10%
Credit Card \| Florida
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	4,794	4,325
Percentage of portfolio	5.60%	5.30%
Credit Card \| Illinois
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	3,747	3,603
Percentage of portfolio	4.40%	4.40%
Credit Card \| Pennsylvania
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	3,581	3,442
Percentage of portfolio	4.20%	4.20%
Credit Card \| Ohio
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	3,075	2,965
Percentage of portfolio	3.60%	3.60%
Credit Card \| New Jersey
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	2,868	2,736
Percentage of portfolio	3.30%	3.40%
Credit Card \| Michigan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	2,681	2,595
Percentage of portfolio	3.10%	3.20%
Credit Card \| Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	37,392	35,379
Percentage of portfolio	43.50%	43.60%
Credit Card \| Domestic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	77,704	73,255
Percentage of portfolio	90.50%	90.10%
Amount, 30 plus day delinquencies	2,538	2,514
Amount, 90 plus day delinquencies	1,181	1,187
Credit Card \| Canada
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	4,747	4,503
Percentage of portfolio	5.50%	5.50%
Credit Card \| United Kingdom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	3,425	3,547
Percentage of portfolio	4.00%	4.40%
Credit Card \| International
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment	8,172	8,050
Percentage of portfolio	9.50%	9.90%
Amount, 30 plus day delinquencies	294	367
Amount, 90 plus day delinquencies	$ 111	$ 141

Financing Receivable, Percent Equal to Greater than 90 Days Past Due

No definition available.

Name:	cof_FinancingReceivablePercentEqualtoGreaterthan90DaysPastDue
Namespace Prefix:	cof_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Financing Receivable, Percent Past Due

+ References

No definition available.

+ Details

Name:	cof_FinancingReceivablePercentPastDue
Namespace Prefix:	cof_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 21

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13537-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

-SubTopic 10

-Section 50

-Paragraph 20

-URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6404-108592

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 20

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13531-108611

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

-SubTopic 10

-Section 50

-Paragraph 18

-URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6351-108592

+ Details

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Financing receivables that are equal to or greater than 90 days past due.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 7A

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953401-111524

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Recorded Investment in financing receivables that are past due at the balance sheet date.

+ References

+ Details

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

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522

+ Details

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

- Details

Name:	cof_FinancingReceivablePortfolioSegmentAxis=cof_CreditCardPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: