Deposits and Borrowings - Long-Term Debt (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term Debt and Capital Lease Obligations	$ 47,393	$ 59,468
Debt and Capital Lease Obligations	$ 48,439	60,460
Securitized debt obligations
Debt Instrument [Line Items]
Weighted average interest rate	1.83%
Long-term debt	$ 18,528	18,826
Total senior and subordinated notes
Debt Instrument [Line Items]
Long-term debt	$ 26,405	23,431
Senior notes \| Total unsecured senior debt
Debt Instrument [Line Items]
Weighted average interest rate	2.64%
Long-term debt	$ 21,877	18,899
Senior notes \| Fixed unsecured senior debt
Debt Instrument [Line Items]
Weighted average interest rate	2.71%
Long-term debt	$ 19,628	17,546
Senior notes \| Floating unsecured senior debt
Debt Instrument [Line Items]
Weighted average interest rate	1.98%
Long-term debt	$ 2,249	1,353
Subordinated notes \| Fixed unsecured subordinated debt
Debt Instrument [Line Items]
Weighted average interest rate	4.09%
Long-term debt	$ 4,528	4,532
FHLB advances
Debt Instrument [Line Items]
Weighted average interest rate	0.75%
Long-term debt	$ 2,428	17,179
Capital Lease Obligations
Debt Instrument [Line Items]
Weighted average interest rate	4.16%
Long-term debt	$ 32	32
FHLB Advance and Capital Lease Obligation
Debt Instrument [Line Items]
Long-term debt	$ 2,460	$ 17,211
Minimum [Member] \| Securitized debt obligations
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	0.95%
Minimum [Member] \| Senior notes \| Fixed unsecured senior debt
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	1.30%
Minimum [Member] \| Senior notes \| Floating unsecured senior debt
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	1.71%
Minimum [Member] \| Subordinated notes \| Fixed unsecured subordinated debt
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.38%
Minimum [Member] \| FHLB advances
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	0.70%
Minimum [Member] \| Capital Lease Obligations
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.09%
Maximum [Member] \| Securitized debt obligations
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.75%
Maximum [Member] \| Senior notes \| Fixed unsecured senior debt
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.75%
Maximum [Member] \| Senior notes \| Floating unsecured senior debt
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	2.19%
Maximum [Member] \| Subordinated notes \| Fixed unsecured subordinated debt
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	8.80%
Maximum [Member] \| FHLB advances
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.41%
Maximum [Member] \| Capital Lease Obligations
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	12.86%

Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.13,16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_DebtAndCapitalLeaseObligations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt and capital lease obligation due after one year or beyond the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebtAndCapitalLeaseObligations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Weighted average interest rate of long-term debt outstanding.

+ References

No definition available.

+ Details

Name:	us-gaap_LongtermDebtWeightedAverageInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=cof_SeniorAndSubordinatedNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cof_UnsecuredSeniorDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cof_FixedUnsecuredSeniorDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cof_FloatingUnsecuredSeniorDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SubordinatedDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cof_FixedUnsecuredSubordinatedDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_FederalHomeLoanBankAdvancesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_CapitalLeaseObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=cof_FHLBAdvanceandCapitalLeaseObligationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: