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Commitments, Contingent Liabilities and Guarantees - Commitments to Extend Credit or Funding (Details) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Guarantor Obligations [Line Items] |  |
Total loan commitments | $ 64,789 | $ 61,478
Commercial letters of credit | 91 | 86
Purchase card commitments | 729 | 621
Principal investing commitments | 21 | 26
Tax credit investment commitments | 547 | 520
Total loan and other commitments | 66,177 | 62,731
Commercial and Other [Member] |  |
Guarantor Obligations [Line Items] |  |
Total loan commitments | 45,323 | 42,653
Commercial Real Estate and Construction [Member] |  |
Guarantor Obligations [Line Items] |  |
Total loan commitments | 2,961 | 2,691
Home Equity [Member] |  |
Guarantor Obligations [Line Items] |  |
Total loan commitments | 9,945 | 9,982
Credit Cards [Member] |  |
Guarantor Obligations [Line Items] |  |
Total loan commitments | $ 6,560 | $ 6,152

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- Definition Commercial Letters Of Credit + References No definition available. + Details
Name: | key_CommercialLettersOfCredit
Namespace Prefix: | key_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Loan and Other Commitments + References No definition available. + Details
Name: | key_LoanAndOtherCommitments
Namespace Prefix: | key_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Loan Commitments + References No definition available. + Details
Name: | key_LoanCommitments
Namespace Prefix: | key_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Principal Investing Commitments + References No definition available. + Details
Name: | key_PrincipalInvestingCommitments
Namespace Prefix: | key_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Purchase Card Commitments + References No definition available. + Details
Name: | key_PurchaseCardCommitments
Namespace Prefix: | key_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Tax Credit Investment Commitments + References No definition available. + Details
Name: | key_TaxCreditInvestmentCommitments
Namespace Prefix: | key_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_FinancialInstrumentAxis=key_CommercialRealEstateAndConstructionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_HomeEquityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_FinancialInstrumentAxis=key_CreditCardsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |