Fair Value Measurements - Change in Fair Values of Level 3 Financial Instruments (Detail) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Securities Available for Sale [Member] \| Other Debt Obligations [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	$ 20,000,000	$ 17,000,000	$ 20,000,000	$ 17,000,000
Gains (Losses) Included in Other Comprehensive Income	0	3,000,000	0	3,000,000
Gains (Losses) Included in Earnings	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Transfers Other	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending Balance	20,000,000	20,000,000	20,000,000	20,000,000
Unrealized Gains (Losses) Included in Earnings	0	0	0	0
Other Investments [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	7,000,000		3,000,000
Gains (Losses) Included in Other Comprehensive Income	0		0
Gains (Losses) Included in Earnings	0		0
Purchases	0		0
Sales	0		0
Settlements	0		0
Transfers Other	0		0
Transfers into Level 3	0		4,000,000
Transfers out of Level 3	0		0
Ending Balance	7,000,000		7,000,000
Unrealized Gains (Losses) Included in Earnings	0		0
Other Investments [Member] \| Principal Investments Direct [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	12,000,000	21,000,000	13,000,000	27,000,000
Gains (Losses) Included in Other Comprehensive Income	0	0	0	0
Gains (Losses) Included in Earnings	1,000,000	(3,000,000)	0	(5,000,000)
Purchases	1,000,000	0	1,000,000	0
Sales	(1,000,000)	(3,000,000)	(1,000,000)	(7,000,000)
Settlements	0	0	0	0
Transfers Other	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending Balance	13,000,000	15,000,000	13,000,000	15,000,000
Unrealized Gains (Losses) Included in Earnings	2,000,000	3,000,000	1,000,000	(3,000,000)
Interest Rate [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	4,000,000	5,000,000	9,000,000	7,000,000
Gains (Losses) Included in Other Comprehensive Income	0	0	0	0
Gains (Losses) Included in Earnings	(1,000,000)	0	(2,000,000)	0
Purchases	0	0	1,000,000	0
Sales	(2,000,000)	0	(2,000,000)	0
Settlements	0	0	0	0
Transfers Other	0	0	0	0
Transfers into Level 3	4,000,000	11,000,000	4,000,000	12,000,000
Transfers out of Level 3	0	0	(5,000,000)	(3,000,000)
Ending Balance	5,000,000	16,000,000	5,000,000	16,000,000
Unrealized Gains (Losses) Included in Earnings	0	0	0	0
Credit [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	0	1,000,000	1,000,000	1,000,000
Gains (Losses) Included in Other Comprehensive Income	0	0	0	0
Gains (Losses) Included in Earnings	(15,000,000)	(4,000,000)	(20,000,000)	(7,000,000)
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	6,000,000	4,000,000	10,000,000	7,000,000
Transfers Other	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending Balance	(9,000,000)	1,000,000	(9,000,000)	1,000,000
Unrealized Gains (Losses) Included in Earnings	0	0	0	0
Other [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	3,000,000	4,000,000	3,000,000	2,000,000
Gains (Losses) Included in Other Comprehensive Income	0	0	0	0
Gains (Losses) Included in Earnings	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Transfers Other	0	0	0	2,000,000
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending Balance	3,000,000	4,000,000	3,000,000	4,000,000
Unrealized Gains (Losses) Included in Earnings	0	$ 0	0	$ 0
Loans Held For Sale Fair Value [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	1,000,000		1,000,000
Gains (Losses) Included in Other Comprehensive Income	0		0
Gains (Losses) Included in Earnings	0		0
Purchases	0		0
Sales	(1,000,000)		(1,000,000)
Settlements	0		0
Transfers Other	0		0
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Ending Balance	0		0
Unrealized Gains (Losses) Included in Earnings	0		0
Loans Held For Investment Fair Value [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	2,000,000		2,000,000
Gains (Losses) Included in Other Comprehensive Income	0		0
Gains (Losses) Included in Earnings	0		0
Purchases	0		0
Sales	0		0
Settlements	0		0
Transfers Other	1,000,000		1,000,000
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Ending Balance	3,000,000		3,000,000
Unrealized Gains (Losses) Included in Earnings	$ 0		$ 0

Fair value measurement with unobservable inputs reconciliation recurring basis asset and liabilities value.

No definition available.

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

- Definition

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Other Transfers

+ References

No definition available.

+ Details

Name:	key_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisOtherTransfers
Namespace Prefix:	key_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of purchases, (sales), issuances and (settlements) of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchasesSalesIssuancesSettlements
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) recognized on the income statement for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_InformationByCategoryOfDebtSecurityAxis=us-gaap_AvailableforsaleSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=key_PrincipalDirectInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_CreditRiskContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_OtherCreditDerivativesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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