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Asset Quality - Changes in Liability for Credit Losses on Lending Related Commitments (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Financing Receivable, Allowance for Credit Loss [Roll Forward] |  |  |
Balance at beginning of period | $ 64 | $ 57 | $ 55
Provision (credit) for losses on lending-related commitments | 4 |  | 2
Balance at end of period | 68 | 64 | 57
Previously Reported [Member] |  |  |
Financing Receivable, Allowance for Credit Loss [Roll Forward] |  |  |
Balance at beginning of period | $ 63 | 57 |
Provision (credit) for losses on lending-related commitments |  | 6 |
Balance at end of period |  | $ 63 | $ 57

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- Definition Financing receivable liability for credit losses. + References No definition available. + Details
Name: | key_FinancingReceivableLiabilityForCreditLosses
Namespace Prefix: | key_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Provision credit for loan and lease losses. + References No definition available. + Details
Name: | key_ProvisionCreditForLoanAndLeaseLosses
Namespace Prefix: | key_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

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- Details
Name: | srt_RestatementAxis=srt_ScenarioPreviouslyReportedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |