Fair Value Measurement - Schedule of Assets Measured at Fair Value on Recurring Basis Quantitative Information about Level 3 Fair Value Measurements (Detail) $ in Millions	Dec. 31, 2016 USD ($) $ / SecurityLoan	Dec. 31, 2015 USD ($) $ / SecurityLoan
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Securities available for sale	$ 40,737	$ 39,061
Derivative Assets, Gross Amount	1,494	1,518
Consumer MSRs	80	68
Fair Value, Measurements, Recurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Securities available for sale	40,737	39,061
Derivative Assets, Gross Amount	1,494	1,518
Fair Value, Measurements, Recurring [Member] \| RMBS
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Securities available for sale	29,249	27,311
Fair Value, Measurements, Recurring [Member] \| CMBS
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Securities available for sale	4,988	5,379
Fair Value, Measurements, Recurring [Member] \| Other ABS
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Securities available for sale	714	1,340
Fair Value, Measurements, Recurring [Member] \| Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Securities available for sale	578	615
Derivative Assets, Gross Amount	47	57
Consumer MSRs	80	68
Retained interests in securitizations	201	211
Fair Value, Measurements, Recurring [Member] \| Level 3 \| RMBS
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Securities available for sale	$ 518	$ 504
Fair Value, Measurements, Recurring [Member] \| Level 3 \| RMBS \| Discounted cash flows \| Minimum
Fair Value Inputs [Abstract]
Yield (percent)	0.00%	0.00%
Constant prepayment rate (percent)	0.00%	0.00%
Default rate (percent)	0.00%	0.00%
Loss severity (percent)	9.00%	16.00%
Fair Value, Measurements, Recurring [Member] \| Level 3 \| RMBS \| Discounted cash flows \| Maximum
Fair Value Inputs [Abstract]
Yield (percent)	15.00%	12.00%
Constant prepayment rate (percent)	30.00%	28.00%
Default rate (percent)	16.00%	8.00%
Loss severity (percent)	87.00%	85.00%
Fair Value, Measurements, Recurring [Member] \| Level 3 \| RMBS \| Discounted cash flows \| Weighted Average
Fair Value Inputs [Abstract]
Yield (percent)	5.00%	6.00%
Constant prepayment rate (percent)	4.00%	4.00%
Default rate (percent)	4.00%	4.00%
Loss severity (percent)	57.00%	55.00%
Fair Value, Measurements, Recurring [Member] \| Level 3 \| CMBS
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Securities available for sale	$ 51	$ 97
Fair Value, Measurements, Recurring [Member] \| Level 3 \| CMBS \| Discounted cash flows \| Minimum
Fair Value Inputs [Abstract]
Yield (percent)	2.00%	2.00%
Constant prepayment rate (percent)	0.00%	0.00%
Fair Value, Measurements, Recurring [Member] \| Level 3 \| CMBS \| Discounted cash flows \| Maximum
Fair Value Inputs [Abstract]
Yield (percent)	2.00%	3.00%
Constant prepayment rate (percent)	0.00%	15.00%
Fair Value, Measurements, Recurring [Member] \| Level 3 \| CMBS \| Discounted cash flows \| Weighted Average
Fair Value Inputs [Abstract]
Yield (percent)	2.00%	3.00%
Constant prepayment rate (percent)	0.00%	9.00%
Fair Value, Measurements, Recurring [Member] \| Level 3 \| Other ABS
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Securities available for sale	$ 0	$ 0
Fair Value, Measurements, Recurring [Member] \| Level 3 \| Foreign government bonds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Securities available for sale	$ 9	$ 14
Fair Value, Measurements, Recurring [Member] \| Level 3 \| Foreign government bonds \| Discounted cash flows \| Minimum
Fair Value Inputs [Abstract]
Yield (percent)	1.00%	1.00%
Fair Value, Measurements, Recurring [Member] \| Level 3 \| Foreign government bonds \| Discounted cash flows \| Maximum
Fair Value Inputs [Abstract]
Yield (percent)	2.00%	1.00%
Fair Value, Measurements, Recurring [Member] \| Level 3 \| Foreign government bonds \| Discounted cash flows \| Weighted Average
Fair Value Inputs [Abstract]
Yield (percent)	1.00%	1.00%
Fair Value, Measurements, Recurring [Member] \| Level 3 \| Derivative assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Assets, Gross Amount	$ 47	$ 57
Fair Value, Measurements, Recurring [Member] \| Level 3 \| Derivative assets \| Discounted cash flows \| Minimum
Fair Value Inputs [Abstract]
Swap rates (percent)	2.00%	2.00%
Fair Value, Measurements, Recurring [Member] \| Level 3 \| Derivative assets \| Discounted cash flows \| Maximum
Fair Value Inputs [Abstract]
Swap rates (percent)	2.00%	2.00%
Fair Value, Measurements, Recurring [Member] \| Level 3 \| Derivative assets \| Discounted cash flows \| Weighted Average
Fair Value Inputs [Abstract]
Swap rates (percent)	2.00%	2.00%
Fair Value, Measurements, Recurring [Member] \| Level 3 \| Consumer MSRs
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Consumer MSRs	$ 80	$ 68
Fair Value, Measurements, Recurring [Member] \| Level 3 \| Consumer MSRs \| Discounted cash flows \| Minimum
Fair Value Inputs [Abstract]
Total prepayment rate (percent)	8.00%	11.00%
Discount rate (percent)	15.00%	12.00%
Option adjusted spread rate (percent)	5.80%	4.35%
Servicing cost ($ per loan) \| $ / SecurityLoan	75	93
Fair Value, Measurements, Recurring [Member] \| Level 3 \| Consumer MSRs \| Discounted cash flows \| Maximum
Fair Value Inputs [Abstract]
Total prepayment rate (percent)	20.00%	18.00%
Discount rate (percent)	15.00%	12.00%
Option adjusted spread rate (percent)	15.00%	15.00%
Servicing cost ($ per loan) \| $ / SecurityLoan	100	201
Fair Value, Measurements, Recurring [Member] \| Level 3 \| Consumer MSRs \| Discounted cash flows \| Weighted Average
Fair Value Inputs [Abstract]
Total prepayment rate (percent)	15.00%	16.00%
Discount rate (percent)	15.00%	12.00%
Option adjusted spread rate (percent)	6.36%	4.74%
Servicing cost ($ per loan) \| $ / SecurityLoan	76	98
Fair Value, Measurements, Recurring [Member] \| Level 3 \| Retained interest in securitizations
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Retained interests in securitizations	$ 201	$ 211
Fair Value, Measurements, Recurring [Member] \| Level 3 \| Retained interest in securitizations \| Discounted cash flows \| Minimum
Fair Value Inputs [Abstract]
Life of receivables (months)	6 months	16 months
Constant prepayment rate (percent)	2.00%	1.00%
Default rate (percent)	1.00%	2.00%
Loss severity (percent)	7.00%	15.00%
Discount rate (percent)	4.00%	4.00%
Fair Value, Measurements, Recurring [Member] \| Level 3 \| Retained interest in securitizations \| Discounted cash flows \| Maximum
Fair Value Inputs [Abstract]
Life of receivables (months)	87 months	75 months
Constant prepayment rate (percent)	11.00%	13.00%
Default rate (percent)	6.00%	6.00%
Loss severity (percent)	102.00%	94.00%
Discount rate (percent)	11.00%	9.00%

Interest rate correlation, used as an input to measure fair value.

No definition available.

Name:	cof_FairValueInputsInterestRateCorrelation
Namespace Prefix:	cof_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair Value Inputs, Life of Receivables

+ References

No definition available.

+ Details

Name:	cof_FairValueInputsLifeofReceivables
Namespace Prefix:	cof_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair Value Inputs, Option Adjusted Spread Rate

+ References

No definition available.

+ Details

Name:	cof_FairValueInputsOptionAdjustedSpreadRate
Namespace Prefix:	cof_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value inputs, swap rate.

+ References

No definition available.

+ Details

Name:	cof_FairValueInputsSwapRate
Namespace Prefix:	cof_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Rate at which loans or a loan portfolio are expected to prepay principal balances with consideration of defaults, used as an input to measure fair value

+ References

No definition available.

+ Details

Name:	cof_FairValueInputsTotalPrepaymentRate
Namespace Prefix:	cof_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value Inputs, Servicing Cost Per Loan

+ References

No definition available.

+ Details

Name:	cof_FairvalueInputsServicingCostPerLoan
Namespace Prefix:	cof_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

+ Details

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

- Definition

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and elected not to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

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

- References

No definition available.

+ Details

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

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

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

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

- Definition

Loss, including interest and principal, incurred on a defaulted security, expressed as a percentage of the original principal balance, used as an input to measure fair value.

+ References

+ Details

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

- Definition

Rate at which loans or a loan portfolio are expected to prepay principal balances, used as an input to measure fair value.

+ References

+ Details

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

- Definition

Percentage of likelihood a loan will not be repaid and instead default, used as an input to measure fair value.

+ References

+ Details

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

- Definition

Fair value portion of interest continued to be held by a transferor after transferring financial assets to a third party.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 10
-Section 15
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=68074228&loc=d3e94415-111705

+ Details

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

- Definition

Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746

+ Details

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

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=cof_OtherAssetbackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=us-gaap_IncomeApproachValuationTechniqueMember
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:

- Details

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

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_DerivativeFinancialInstrumentsAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_ServicingContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_RetainedInterestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: