Leases - Summary of Additional Balance Sheet Information (Details) $ in Millions	Dec. 31, 2019 USD ($)
Leases [Abstract]
Operating lease, right-of-use assets	$ 654
Operating lease, lease liabilities	748
Finance leases:
Property and equipment, gross	28
Accumulated depreciation	(17)
Property and equipment, net	11
Finance lease liabilities	$ 13

Finance Lease, RIght-Of-Use Asset [Abstract]

No definition available.

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- Definition

Finance Lease, Right-Of-Use Asset, Accumulated Depreciation

+ References

No definition available.

+ Details

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- Definition

Finance Lease, Right-Of-Use Asset, Gross

+ References

No definition available.

+ Details

Name:	key_FinanceLeaseRightOfUseAssetGross
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

Present value of lessee's discounted obligation for lease payments from finance lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918701-209980

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of lessee's right to use underlying asset under finance lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

+ Details

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Balance Type:	debit
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- References

No definition available.

+ Details

Name:	us-gaap_LeasesAbstract
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- Definition

Present value of lessee's discounted obligation for lease payments from operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

Amount of lessee's right to use underlying asset under operating lease.

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant