Shareholders' Equity - Key's and KeyBank's Actual Capital Amounts and Ratios, Minimum Capital Amounts and Ratios (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Key [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Net Risk-Weighted Assets, Actual Amount	$ 15,345	$ 15,638
Tier 1 Capital to Average Quarterly Tangible Assets, Amount	13,083	13,249
Tier 1 Capital to Net Risk-Weighted Assets, Actual Amount	$ 13,083	$ 13,249
Total Capital to Net Risk-Weighted Assets, Actual Ratio	12.92%	12.85%
Tier 1 Capital to Net Risk-Weighted Assets, Actual Ratio	11.01%	10.89%
Tier 1 Capital to Average Quarterly Tangible Assets, Actual Ratio	9.73%	9.90%
Total Capital to Net Risk-Weighted Assets, to Meet Minimum Capital Adequacy Requirements, Amount	$ 9,505	$ 9,734
Tier 1 Capital to Net Risk-Weighted Assets, to Meet Minimum Capital Adequacy Requirements, Amount	7,129	7,300
Tier 1 Capital to Average Quarterly Tangible Assets to Meet Minimum Capital Adequacy Requirements, Amount	$ 5,379	$ 5,442
Total Capital to Net Risk-Weighted Assets, to Meet Minimum Capital Adequacy Requirements, Ratio	8.00%	8.00%
Tier 1 Capital to Net Risk-Weighted Assets, to Meet Minimum Capital Adequacy Requirements, Ratio	6.00%	6.00%
Tier 1 Capital to Average Quarterly Tangible Assets to Meet Minimum Capital Adequacy Requirements, Ratio	4.00%	4.00%
KeyBank (consolidated) [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Net Risk-Weighted Assets, Actual Amount	$ 14,957	$ 14,291
Tier 1 Capital to Average Quarterly Tangible Assets, Amount	13,110	12,439
Tier 1 Capital to Net Risk-Weighted Assets, Actual Amount	$ 13,110	$ 12,439
Total Capital to Net Risk-Weighted Assets, Actual Ratio	12.86%	12.36%
Tier 1 Capital to Net Risk-Weighted Assets, Actual Ratio	11.27%	10.76%
Tier 1 Capital to Average Quarterly Tangible Assets, Actual Ratio	9.91%	9.46%
Total Capital to Net Risk-Weighted Assets, to Meet Minimum Capital Adequacy Requirements, Amount	$ 9,306	$ 9,251
Tier 1 Capital to Net Risk-Weighted Assets, to Meet Minimum Capital Adequacy Requirements, Amount	6,980	6,939
Tier 1 Capital to Average Quarterly Tangible Assets to Meet Minimum Capital Adequacy Requirements, Amount	$ 5,293	$ 5,353
Total Capital to Net Risk-Weighted Assets, to Meet Minimum Capital Adequacy Requirements, Ratio	8.00%	8.00%
Tier 1 Capital to Net Risk-Weighted Assets, to Meet Minimum Capital Adequacy Requirements, Ratio	6.00%	6.00%
Tier 1 Capital to Average Quarterly Tangible Assets to Meet Minimum Capital Adequacy Requirements, Ratio	4.00%	4.00%
Total Capital to Net Risk-Weighted Assets, to Qualify as Well Capitalized Under Federal Deposit Insurance Act, Amount	$ 11,633	$ 11,564
Tier 1 Capital to Net Risk-Weighted Assets, to Qualify as Well Capitalized Under Federal Deposit Insurance Act, Amount	6,980	6,939
Tier 1 Capital to Average Quarterly Tangible Assets to Qualify as Well Capitalized Under Federal Deposit Insurance Act, Amount	$ 6,617	$ 6,691
Total Capital to Net Risk-Weighted Assets, to Qualify as Well Capitalized Under Federal Deposit Insurance Act, Ratio	10.00%	10.00%
Tier 1 Capital to Net Risk-Weighted Assets, to Qualify as Well Capitalized Under Federal Deposit Insurance Act, Ratio	6.00%	6.00%
Tier 1 Capital to Average Quarterly Tangible Assets to Qualify as Well Capitalized Under Federal Deposit Insurance Act, Ratio	5.00%	5.00%

Tangible Capital Qualify As Well Capitalized Under Federal Deposit Insurance Act

No definition available.

Name:	key_TangibleCapitalQualifyAsWellCapitalizedUnderFederalDepositInsuranceAct
Namespace Prefix:	key_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tangible Capital Qualify As Well Capitalized Under Federal Deposit Insurance Act Ratio

+ References

No definition available.

+ Details

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

- Definition

The amount of total capital as defined in the regulations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6490092&loc=d3e47304-110998

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1H
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1H
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=6490092&loc=d3e47080-110998

+ Details

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

- Definition

The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1H
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826

+ Details

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

- Definition

The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1H
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958570-112826

+ Details

Name:	us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826

+ Details

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

- Definition

The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Total Capital divided by risk weighted assets as defined in the regulations.

+ References

+ Details

Name:	us-gaap_CapitalToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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

Name:	us-gaap_ComplianceWithRegulatoryCapitalRequirementsUnderBankingRegulationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Tangible capital as defined in the regulations (generally includes outstanding stock plus retained earnings) for banking institutions subject to Office of Thrift Supervision (OTS) capital requirements.

+ References

+ Details

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

- Definition

The minimum amount of tangible capital required under the regulatory framework for prompt corrective action.

+ References

+ Details

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

- Definition

The minimum amount of Tangible Capital divided by tangible assets (Tangible Capital Ratio) as defined in the regulations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958570-112826

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1H
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826

+ Details

Name:	us-gaap_TangibleCapitalRequiredForCapitalAdequacyToTangibleAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Tangible Capital divided by Tangible assets as defined in the regulations.

+ References

+ Details

Name:	us-gaap_TangibleCapitalToTangibleAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Tier 1 Risk Based Capital as defined in the regulations.

+ References

+ Details

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

- Definition

The minimum amount of Tier 1 Risk Based Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

+ Details

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

- Definition

The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958570-112826

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1H
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

+ References

+ Details

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

- Definition

The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Tier 1 capital divided by risk weighted assets as defined by regulations.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_ConsolidatedEntitiesAxis=us-gaap_ParentCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=key_BankingSubsidiaryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: