Investment Securities - Schedule of Expected Maturities and Weighted Average Yields of Investment Securities by Major Security Type (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Securities available for sale
Due in 1 Year or Less	$ 1,170
Due 1 Year through 5 Years	17,068
Due 5 Years through 10 Years	22,244
Due 10 Years	638
Available-for-sale Securities	41,120	$ 40,737
Amortized cost of securities available for sale
Due in 1 Year or Less	1,169
Due 1 Year through 5 Years	16,884
Due 5 Years through 10 Years	22,186
Due 10 Years	594
Total Amortized Cost	$ 40,833	40,733
Weighted average yield for securities available for sale
Due in 1 Year or Less	1.36%
Due 1 Year through 5 Years	2.45%
Due 5 Years through 10 Years	2.45%
Due 10 Years	4.96%
Total weighted average yield	2.45%
Carrying value of securities held to maturity
Due in 1 Year or Less	$ 227
Due 1 Year through 5 Years	2,427
Due 5 Years through 10 Years	19,608
Due 10 Years	5,458
Carrying Value	27,720	25,712
Fair value of securities held to maturity
Due in 1 Year or Less	225
Due 1 Year through 5 Years	2,501
Due 5 Years through 10 Years	20,092
Due 10 Years	5,605
Fair Value	$ 28,423	26,196
Weighted average yield for securities held to maturity
Due in 1 Year or Less	0.60%
Due 1 Year through 5 Years	2.57%
Due 5 Years through 10 Years	2.68%
Due 10 Years	3.36%
Total weighted average yield	2.78%
U.S. Treasury securities
Carrying value of securities held to maturity
Due in 1 Year or Less	$ 199
Due 1 Year through 5 Years	0
Due 5 Years through 10 Years	0
Due 10 Years	0
Carrying Value	199	199
Fair value of securities held to maturity
Fair Value	199	199
U.S. Treasury securities
Securities available for sale
Due in 1 Year or Less	102
Due 1 Year through 5 Years	847
Due 5 Years through 10 Years	4,266
Due 10 Years	0
Available-for-sale Securities	5,215	5,065
Amortized cost of securities available for sale
Total Amortized Cost	5,218	5,103
RMBS
Securities available for sale
Due in 1 Year or Less	195
Due 1 Year through 5 Years	12,779
Due 5 Years through 10 Years	15,859
Due 10 Years	295
Available-for-sale Securities	29,128	29,249
Amortized cost of securities available for sale
Total Amortized Cost	28,856	29,179
RMBS \| Agency
Securities available for sale
Due in 1 Year or Less	177
Due 1 Year through 5 Years	11,743
Due 5 Years through 10 Years	14,591
Due 10 Years	0
Available-for-sale Securities	26,511	26,527
Amortized cost of securities available for sale
Total Amortized Cost	26,693	26,830
Carrying value of securities held to maturity
Due in 1 Year or Less	28
Due 1 Year through 5 Years	1,608
Due 5 Years through 10 Years	17,865
Due 10 Years	4,409
Carrying Value	23,910	22,125
Fair value of securities held to maturity
Fair Value	24,537	22,573
RMBS \| Non-Agency
Securities available for sale
Due in 1 Year or Less	18
Due 1 Year through 5 Years	1,036
Due 5 Years through 10 Years	1,268
Due 10 Years	295
Available-for-sale Securities	2,617	2,722
Amortized cost of securities available for sale
Total Amortized Cost	2,163	2,349
CMBS
Securities available for sale
Due in 1 Year or Less	309
Due 1 Year through 5 Years	2,608
Due 5 Years through 10 Years	2,009
Due 10 Years	0
Available-for-sale Securities	4,926	4,988
Amortized cost of securities available for sale
Total Amortized Cost	4,913	5,011
CMBS \| Agency
Securities available for sale
Due in 1 Year or Less	130
Due 1 Year through 5 Years	1,544
Due 5 Years through 10 Years	1,450
Due 10 Years	0
Available-for-sale Securities	3,124	3,304
Amortized cost of securities available for sale
Total Amortized Cost	3,136	3,335
Carrying value of securities held to maturity
Due in 1 Year or Less	0
Due 1 Year through 5 Years	819
Due 5 Years through 10 Years	1,743
Due 10 Years	1,049
Carrying Value	3,611	3,388
Fair value of securities held to maturity
Fair Value	3,687	3,424
CMBS \| Non-Agency
Securities available for sale
Due in 1 Year or Less	179
Due 1 Year through 5 Years	1,064
Due 5 Years through 10 Years	559
Due 10 Years	0
Available-for-sale Securities	1,802	1,684
Amortized cost of securities available for sale
Total Amortized Cost	1,777	1,676
Other ABS
Securities available for sale
Due in 1 Year or Less	348
Due 1 Year through 5 Years	272
Due 5 Years through 10 Years	7
Due 10 Years	0
Available-for-sale Securities	627	714
Amortized cost of securities available for sale
Total Amortized Cost	626	714
Other securities
Securities available for sale
Due in 1 Year or Less	216
Due 1 Year through 5 Years	562
Due 5 Years through 10 Years	103
Due 10 Years	343
Available-for-sale Securities	1,224	721
Amortized cost of securities available for sale
Total Amortized Cost	$ 1,220	$ 726

Available-for-sale Securities, Expected Debt Maturities, Amortized Cost Basis, Rolling Maturity

No definition available.

Name:	cof_AvailableforsaleSecuritiesExpectedDebtMaturitiesAmortizedCostBasisRollingMaturity
Namespace Prefix:	cof_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Available-for-sale Securities, Expected Debt Maturities, Fair Value, Rolling Maturity [Abstract]

+ References

No definition available.

+ Details

Name:	cof_AvailableforsaleSecuritiesExpectedDebtMaturitiesFairValueRollingMaturityAbstract
Namespace Prefix:	cof_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Available-for-sale Securities, Expected Debt Maturities, Next Rolling Twelve Months, Amortized Cost Basis

+ References

No definition available.

+ Details

Name:	cof_AvailableforsaleSecuritiesExpectedDebtMaturitiesNextRollingTwelveMonthsAmortizedCostBasis
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Available-for-sale Securities, Expected Debt Maturities, Next Rolling Twelve Months, Fair Value

+ References

No definition available.

+ Details

Name:	cof_AvailableforsaleSecuritiesExpectedDebtMaturitiesNextRollingTwelveMonthsFairValue
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Available-for-sale Securities, Expected Debt Maturities, Next Rolling Twelve Months, Weighted Average Yield

+ References

No definition available.

+ Details

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

- Definition

Available-for-sale Securities, Expected Debt Maturities, Rolling Year Six Through Ten, Amortized Cost Basis

+ References

No definition available.

+ Details

Name:	cof_AvailableforsaleSecuritiesExpectedDebtMaturitiesRollingYearSixThroughTenAmortizedCostBasis
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Available-for-sale Securities, Expected Debt Maturities, Rolling Year Six Through Ten, Fair Value

+ References

No definition available.

+ Details

Name:	cof_AvailableforsaleSecuritiesExpectedDebtMaturitiesRollingYearSixThroughTenFairValue
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Available-for-sale Securities, Expected Debt Maturities, Rolling Year Six Through Ten, Weighted Average Yield

+ References

No definition available.

+ Details

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

- Definition

Available-for-sale Securities, Expected Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis

+ References

No definition available.

+ Details

Name:	cof_AvailableforsaleSecuritiesExpectedDebtMaturitiesRollingYearTwoThroughFiveAmortizedCostBasis
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Available-for-sale Securities, Expected Debt Maturities, Rolling Year Two Through Five, Fair Value

+ References

No definition available.

+ Details

Name:	cof_AvailableforsaleSecuritiesExpectedDebtMaturitiesRollingYearTwoThroughFiveFairValue
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Available-for-sale Securities, Expected Debt Maturities, Rolling Year Two Through Five, Weighted Average Yield

+ References

No definition available.

+ Details

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

- Definition

Available-for-sale Securities, Expected Debt Maturities, Rolling after Year Ten, Amortized Cost Basis

+ References

No definition available.

+ Details

Name:	cof_AvailableforsaleSecuritiesExpectedDebtMaturitiesRollingafterYearTenAmortizedCostBasis
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Available-for-sale Securities, Expected Debt Maturities, Rolling after Year Ten, Fair Value

+ References

No definition available.

+ Details

Name:	cof_AvailableforsaleSecuritiesExpectedDebtMaturitiesRollingafterYearTenFairValue
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Available-for-sale Securities, Expected Debt Maturities, Rolling after Year Ten, Weighted Average Yield

+ References

No definition available.

+ Details

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

- Definition

Available-for-sale Securities, Weighted Average Yield

+ References

No definition available.

+ Details

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

- Definition

Available-for-sale Securities, Weighted Average Yield [Abstract]

+ References

No definition available.

+ Details

Name:	cof_AvailableforsaleSecuritiesWeightedAverageYieldAbstract
Namespace Prefix:	cof_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Held to maturity securities expected debt maturities weighted average yield

+ References

No definition available.

+ Details

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

- Definition

Held-to-maturity Securities, Expected Debt Maturities, Fair Value, Rolling Maturity [Abstract]

+ References

No definition available.

+ Details

Name:	cof_HeldtomaturitySecuritiesExpectedDebtMaturitiesFairValueRollingMaturityAbstract
Namespace Prefix:	cof_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Held-to-maturity Securities, Expected Debt Maturities, Net Carrying Amount [Abstract]

+ References

No definition available.

+ Details

Name:	cof_HeldtomaturitySecuritiesExpectedDebtMaturitiesNetCarryingAmountAbstract
Namespace Prefix:	cof_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Held-to-maturity Securities, Expected Debt Maturities, Next Rolling Twelve Months, Fair Value

+ References

No definition available.

+ Details

Name:	cof_HeldtomaturitySecuritiesExpectedDebtMaturitiesNextRollingTwelveMonthsFairValue
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Held-to-maturity Securities, Expected Debt Maturities, Next Rolling Twelve Months, Weighted-Average Yield

+ References

No definition available.

+ Details

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

- Definition

Held-to-maturity Securities, Expected Debt Maturities, Rolling Year Six Through Ten, Fair Value

+ References

No definition available.

+ Details

Name:	cof_HeldtomaturitySecuritiesExpectedDebtMaturitiesRollingYearSixThroughTenFairValue
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Held-to-maturity Securities, Expected Debt Maturities, Rolling Year Six Through Ten, Weighted-Average Yield

+ References

No definition available.

+ Details

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

- Definition

Held-to-maturity Securities, Expected Debt Maturities, Rolling Year Two Through Five, Fair Value

+ References

No definition available.

+ Details

Name:	cof_HeldtomaturitySecuritiesExpectedDebtMaturitiesRollingYearTwoThroughFiveFairValue
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Held-to-maturity Securities, Expected Debt Maturities, Rolling Year Two Through Five, Weighted-Average Yield

+ References

No definition available.

+ Details

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

- Definition

Held-to-maturity Securities, Expected Debt Maturities, Rolling after Ten Years, Fair Value

+ References

No definition available.

+ Details

Name:	cof_HeldtomaturitySecuritiesExpectedDebtMaturitiesRollingafterTenYearsFairValue
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Held-to-maturity Securities, Expected Debt Maturities, Rolling after Ten Years, Weighted-Average Yield

+ References

No definition available.

+ Details

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

- Definition

Held-to-maturity Securities, Expected Debt Maturities, Weighted Average Yield [Abstract]

+ References

No definition available.

+ Details

Name:	cof_HeldtomaturitySecuritiesExpectedDebtMaturitiesWeightedAverageYieldAbstract
Namespace Prefix:	cof_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Held-to-maturity Securities, Expected Debt Maturities, after Five Through Ten Years, Net Carrying Amount

+ References

No definition available.

+ Details

Name:	cof_HeldtomaturitySecuritiesExpectedDebtMaturitiesafterFiveThroughTenYearsNetCarryingAmount
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Held-to-maturity Securities, Expected Debt Maturities, after One Through Five Years, Net Carrying Amount

+ References

No definition available.

+ Details

Name:	cof_HeldtomaturitySecuritiesExpectedDebtMaturitiesafterOneThroughFiveYearsNetCarryingAmount
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Held-to-maturity Securities, Expected Debt Maturities, after Ten Years, Net Carrying Amount

+ References

No definition available.

+ Details

Name:	cof_HeldtomaturitySecuritiesExpectedDebtMaturitiesafterTenYearsNetCarryingAmount
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Held-to-maturity Securities, Expected Debt Maturities, within One Year, Net Carrying Amount

+ References

No definition available.

+ Details

Name:	cof_HeldtomaturitySecuritiesExpectedDebtMaturitieswithinOneYearNetCarryingAmount
Namespace Prefix:	cof_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ 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 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.6)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Fair value of investments in debt securities classified as held-to-maturity.

+ References

+ Details

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

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=cof_OtherDebtandEquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: