Securities - Additional Information (Detail) $ in Millions	9 Months Ended
Sep. 30, 2018 USD ($) loan position	Dec. 31, 2017 USD ($)
Debt Securities, Available-for-sale [Line Items]
Loans pledged as collateral	$ 8,400
Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Number of fixed rate collateralized mortgage obligations \| loan	482
Weighted average maturity collateralized mortgage life	4 years 8 months 8 days
Residential Mortgage Backed Securities [Member] \| Agency residential mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	$ 49	$ 20
Securities in unrealized loss positions, number of positions \| position	262
Weighted average maturity mortgage backed securities	4 years 3 months 18 days
Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses		$ 66
Commercial Mortgage Backed Securities [Member] \| Agency residential mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	$ 123
Securities in unrealized loss positions, number of positions \| position	20
Weighted average maturity mortgage backed securities	4 years 1 month 24 days

Number of fixed-rate collateralized mortgage obligations.

No definition available.

Name:	key_NumberOfFixedRateCollateralizedMortgageObligations
Namespace Prefix:	key_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted average maturity mortgage backed securities in years.

+ References

No definition available.

+ Details

Name:	key_WeightedAverageMaturityCollateralizedMortgageLife
Namespace Prefix:	key_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted Average Maturity Mortgage Backed Securities

+ References

No definition available.

+ Details

Name:	key_WeightedAverageMaturityMortgageBackedSecurities
Namespace Prefix:	key_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of loans that have been pledged as collateral for borrowings. If the secured party has the right by contract or custom to sell or re-pledge the collateral, then the debtor reclassifies that asset and report that asset in its statement of financial position separately from other assets not so encumbered.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5086-111524

+ Details

Name:	us-gaap_LoansPledgedAsCollateral
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_CollateralizedMortgageObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=us-gaap_MortgageBackedSecuritiesOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: