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Variable Interest Entities - Variable Interest Entities Information (Details) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Variable Interest Entity [Line Items] |  |
Total Assets | $ 282 | $ 248
Total Liabilities | 1 | 2
LIHTC Investments [Member] |  |
Variable Interest Entity [Line Items] |  |
Total Assets | 6,405 | 5,932
Total Liabilities | 2,526 | 2,569
Maximum Exposure to Loss | 1,846 | 1,740
KCC Indirect Investments [Member] |  |
Variable Interest Entity [Line Items] |  |
Total Assets | 12,954 | 19,659
Total Liabilities | 205 | 376
Maximum Exposure to Loss | $ 89 | $ 122

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- Definition Maximum exposure to loss unconsolidated VIE. + References No definition available. + Details
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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
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Period Type: | duration

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- Definition Carrying amount of the assets in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), where the reporting entity is not the VIE's primary beneficiary. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph a -URI http://asc.fasb.org/extlink&oid=118582676&loc=d3e5728-111685 + Details
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Balance Type: | debit
Period Type: | instant

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- Definition Carrying amount of the liabilities in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), where the reporting entity is not the VIE's primary beneficiary. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph a -URI http://asc.fasb.org/extlink&oid=118582676&loc=d3e5728-111685 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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