Contingent Liabilities and Guarantees - Additional Information (Details) $ in Millions	3 Months Ended
Mar. 31, 2019 USD ($)
Commitments Contingencies And Guarantees [Line Items]
Default guarantees, value	$ 134
Written Put Options [Member]
Commitments Contingencies And Guarantees [Line Items]
Default guarantees, value	$ 46
Weighted average life of written put options	3 years
Standby Letters of Credit [Member]
Commitments Contingencies And Guarantees [Line Items]
Remaining weighted-average life of standby letters of credit in years	2 years
Default guarantees, value	$ 76
Recourse Agreement with FNMA [Member]
Commitments Contingencies And Guarantees [Line Items]
Weighted-average remaining term for outstanding commercial mortgage loans in years	7 years 9 months 18 days
Unpaid principal balance outstanding of loans sold	$ 14,500
Maximum potential amount of undiscounted future payments possibly required as percentage of principal balance of loans outstanding	29.00%
Default guarantees, value	$ 6
Residential Mortgage Reserve [Member]
Commitments Contingencies And Guarantees [Line Items]
Unpaid principal balance outstanding of loans sold	$ 5,300
Maximum potential amount of undiscounted future payments possibly required as percentage of principal balance of loans outstanding	30.00%
Weighted-average loan to value ratio (as a percent)	74.00%
Liability for estimated repurchase obligations on loans sold	$ 6
Minimum [Member] \| Standby Letters of Credit [Member]
Commitments Contingencies And Guarantees [Line Items]
Remaining actual life of standby letters of credit	1 year
Minimum [Member] \| Low [Member]
Commitments Contingencies And Guarantees [Line Items]
Guarantee obligations, percentage	0.00%
Minimum [Member] \| Moderate [Member]
Commitments Contingencies And Guarantees [Line Items]
Guarantee obligations, percentage	30.00%
Minimum [Member] \| High [Member]
Commitments Contingencies And Guarantees [Line Items]
Guarantee obligations, percentage	70.00%
Maximum [Member] \| Standby Letters of Credit [Member]
Commitments Contingencies And Guarantees [Line Items]
Remaining actual life of standby letters of credit	16 years
Maximum [Member] \| Low [Member]
Commitments Contingencies And Guarantees [Line Items]
Guarantee obligations, percentage	30.00%
Maximum [Member] \| Moderate [Member]
Commitments Contingencies And Guarantees [Line Items]
Guarantee obligations, percentage	70.00%

Commitments Contingencies And Guarantees [Line Items]

No definition available.

Name:	key_CommitmentsContingenciesAndGuaranteesLineItems
Namespace Prefix:	key_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Guarantor Obligations, Maximum Exposure, Undiscounted as Percentage of Total Loans Outstanding

+ References

No definition available.

+ Details

Name:	key_GuarantorObligationsMaximumExposureUndiscountedasPercentageofTotalLoansOutstanding
Namespace Prefix:	key_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Remaining actual life letters of credit.

+ References

No definition available.

+ Details

Name:	key_RemainingActualLifeLettersOfCredit
Namespace Prefix:	key_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Remaining weighted-average life of standby letters of credit.

+ References

No definition available.

+ Details

Name:	key_RemainingWeightedAverageLifeOfStandbyLettersOfCredit
Namespace Prefix:	key_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average life of written put options.

+ References

No definition available.

+ Details

Name:	key_WeightedAverageLifeOfWrittenPutOptions
Namespace Prefix:	key_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted Average Loan to Value, Ratio

+ References

No definition available.

+ Details

Name:	key_WeightedAverageLoantoValueRatio
Namespace Prefix:	key_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted-average remaining term for outstanding commercial mortgage loans.

+ References

No definition available.

+ Details

Name:	key_WeightedAverageRemainingTermForOutstandingCommercialMortgageLoans
Namespace Prefix:	key_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(1)(i)(A)(ii))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

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

- Definition

The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248

+ Details

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

- Definition

Indicates, if estimable, (as a percentage) the approximate extent to which the proceeds from liquidation of any assets held either as collateral or by third parties would be expected to cover the maximum potential amount of future payments under the guarantee or each group of similar guarantees.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248

+ Details

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

- Definition

Amount of the recorded obligation to transfer funds in the future for fixed or minimum amounts or quantities of goods or services at fixed or minimum prices (for example, as in take-or-pay contracts or throughput contracts).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 440
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=77896938&loc=d3e25383-109308

+ Details

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

- Details

Name:	us-gaap_OpenOptionContractsWrittenTypeAxis=us-gaap_PutOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=us-gaap_FinancialStandbyLetterOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=key_UnderwritingAndServicingProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=key_ResidentialMortgageReserveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_RiskLevelLowMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_RiskLevelMediumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_RiskLevelHighMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: