Fair Value Measurements - Fair Value of Assets and Liabilities Measured on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
ASSETS MEASURED ON A RECURRING BASIS
Total trading account assets	$ 750us-gaap_TradingSecurities	$ 738us-gaap_TradingSecurities
Securities available for sale	13,360us-gaap_AvailableForSaleSecurities	12,346us-gaap_AvailableForSaleSecurities
Derivative asset netting adjustments	(1,034)	(812)
Total derivative assets	609us-gaap_DerivativeAssets	407us-gaap_DerivativeAssets
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liability netting adjustments	(542)	(500)
Total derivative liabilities	784us-gaap_DerivativeLiabilities	414us-gaap_DerivativeLiabilities
States and Political Subdivisions [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	23	40
Collateralized Mortgage Obligations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	11,270	11,000
Other Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	32	20
Other Mortgage-Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	2,035	1,286
Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Commercial loans	2	3
Total trading account assets	750	738
Securities available for sale	13,360	12,346
Total other investments	416	577
Derivative assets	1,643	1,219
Derivative asset netting adjustments	(1,034)	(812)
Total derivative assets	609	407
Accrued income and other assets		1
Total assets at fair value	15,135	14,069
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities	1,326	914
Derivative liability netting adjustments	(542)	(500)
Total derivative liabilities	784	414
Accrued expense and other liabilities		1
Total liabilities on a recurring basis at fair value	1,207	758
Recurring [Member] \| U.S. Treasury, Agencies and Corporations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account assets	555	471
Recurring [Member] \| States and Political Subdivisions [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account assets	38	23
Securities available for sale	23	40
Recurring [Member] \| Collateralized Mortgage Obligations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account assets		9
Securities available for sale	11,270	11,000
Recurring [Member] \| Other Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account assets	31	112
Securities available for sale	32	20
Recurring [Member] \| Trading Account Assets [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account assets	748	735
Recurring [Member] \| Principal Investments Direct [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	104	141
Recurring [Member] \| Principal Investments Indirect [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	302	413
Recurring [Member] \| Principal Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	406	554
Recurring [Member] \| Equity and Mezzanine Investments Indirect [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	10	23
Recurring [Member] \| Private Equity and Mezzanine Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	10	23
Recurring [Member] \| Other Mortgage-Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account assets	124	120
Securities available for sale	2,035	1,286
Recurring [Member] \| Short Positions [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities	423	343
Recurring [Member] \| Interest Rate [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets	937	1,039
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities	644	739
Recurring [Member] \| Foreign Exchange [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets	93	63
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities	81	57
Recurring [Member] \| Commodity [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets	608	112
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities	594	106
Recurring [Member] \| Credit [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets	5	5
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities	7	12
Level 1 [Member] \| Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account assets	2	4
Securities available for sale	22	20
Total other investments	2
Derivative assets	91	56
Total derivative assets	91	56
Total assets at fair value	117	80
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities	77	49
Total derivative liabilities	77	49
Total liabilities on a recurring basis at fair value	77	51
Level 1 [Member] \| Recurring [Member] \| Other Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account assets	2	4
Securities available for sale	22	20
Level 1 [Member] \| Recurring [Member] \| Trading Account Assets [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account assets	2	4
Level 1 [Member] \| Recurring [Member] \| Principal Investments Direct [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	2
Level 1 [Member] \| Recurring [Member] \| Principal Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	2
Level 1 [Member] \| Recurring [Member] \| Short Positions [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities		2
Level 1 [Member] \| Recurring [Member] \| Foreign Exchange [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets	91	56
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities	77	49
Level 2 [Member] \| Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Commercial loans	2	3
Total trading account assets	748	734
Securities available for sale	13,328	12,326
Derivative assets	1,536	1,134
Total derivative assets	1,536	1,134
Accrued income and other assets		1
Total assets at fair value	15,612	14,195
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities	1,248	864
Total derivative liabilities	1,248	864
Accrued expense and other liabilities		1
Total liabilities on a recurring basis at fair value	1,671	1,206
Level 2 [Member] \| Recurring [Member] \| U.S. Treasury, Agencies and Corporations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account assets	555	471
Level 2 [Member] \| Recurring [Member] \| States and Political Subdivisions [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account assets	38	23
Securities available for sale	23	40
Level 2 [Member] \| Recurring [Member] \| Collateralized Mortgage Obligations [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account assets		9
Securities available for sale	11,270	11,000
Level 2 [Member] \| Recurring [Member] \| Other Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account assets	29	108
Level 2 [Member] \| Recurring [Member] \| Trading Account Assets [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account assets	746	731
Level 2 [Member] \| Recurring [Member] \| Other Mortgage-Backed Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total trading account assets	124	120
Securities available for sale	2,035	1,286
Level 2 [Member] \| Recurring [Member] \| Short Positions [Member]
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities	423	341
Level 2 [Member] \| Recurring [Member] \| Interest Rate [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets	924	1,014
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities	644	739
Level 2 [Member] \| Recurring [Member] \| Foreign Exchange [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets	2	7
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities	4	8
Level 2 [Member] \| Recurring [Member] \| Commodity [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets	608	112
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities	594	106
Level 2 [Member] \| Recurring [Member] \| Credit [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets	2	1
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities	6	11
Level 3 [Member] \| Recurring [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	10
Total other investments	414	577
Derivative assets	16	29
Total derivative assets	16	29
Total assets at fair value	440	606
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities	1	1
Total derivative liabilities	1	1
Total liabilities on a recurring basis at fair value	1	1
Level 3 [Member] \| Recurring [Member] \| Other Securities [Member]
ASSETS MEASURED ON A RECURRING BASIS
Securities available for sale	10
Level 3 [Member] \| Recurring [Member] \| Principal Investments Direct [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	102	141
Level 3 [Member] \| Recurring [Member] \| Principal Investments Indirect [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	302	413
Level 3 [Member] \| Recurring [Member] \| Principal Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	404	554
Level 3 [Member] \| Recurring [Member] \| Equity and Mezzanine Investments Indirect [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	10	23
Level 3 [Member] \| Recurring [Member] \| Private Equity and Mezzanine Investments [Member]
ASSETS MEASURED ON A RECURRING BASIS
Total other investments	10	23
Level 3 [Member] \| Recurring [Member] \| Interest Rate [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets	13	25
Level 3 [Member] \| Recurring [Member] \| Credit [Member]
ASSETS MEASURED ON A RECURRING BASIS
Derivative assets	3	4
LIABILITIES MEASURED ON A RECURRING BASIS
Derivative liabilities	$ 1	$ 1

Commercial loans fair value disclosure.

No definition available.

Name:	key_CommercialLoansFairValueDisclosure
Namespace Prefix:	key_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of trade and related party payables and accrued expenses.

+ References

No definition available.

+ Details

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

- Definition

Fair value of investments in certain entities that calculate net asset value per share, including, but not limited to, by unit, membership interest, or other ownership interest. Investments include, but are not limited to, investments in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds.

+ 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 (a)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 6A

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=SL6742756-110258

+ Details

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

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

+ Details

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

- Details

Name:	us-gaap_AssetsFairValueDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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=28360136&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=27724398&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=6871852&loc=d3e26610-111562

+ Details

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

- Definition

Amount of obligation to return cash collateral under master netting arrangements offset against derivative assets.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 50

-Paragraph 3

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312

+ Details

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

- Definition

Fair value, after the 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 not elected to be offset.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13495-108611

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 50

-Paragraph 3

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 55

-Paragraph 10

-URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226008-175313

+ Details

Name:	us-gaap_DerivativeAssets
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 elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ 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=34720828&loc=SL20225862-175312

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4B

-Subparagraph (a),(c)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 55

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226000-175313

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeAsset
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 liability or 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 liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

+ Details

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

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or 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 liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

+ Details

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

- Definition

Amount of right to receive cash collateral under master netting arrangements offset against derivative liabilities.

+ References

+ Details

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

- Definition

Fair value of financial and nonfinancial obligations.

+ References

+ Details

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

- Details

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

- Definition

Fair value portion of other assets.

+ References

No definition available.

+ Details

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

- Definition

Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

+ References

Reference 1: 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=6871852&loc=d3e26610-111562

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26626-111562

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.4)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
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_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=us-gaap_MortgageBackedSecuritiesOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: