Mortgage Servicing Assets - Schedule of Range and Weighted-Average of Significant Unobservable Inputs (Detail) - Discounted Cash Flow [Member] - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Commercial Mortgage Backed Securities [Member] \| Minimum [Member]
Servicing Assets at Fair Value [Line Items]
Prepayment speed	1.60%	1.70%	1.90%
Expected defaults	1.00%	1.00%	1.00%
Residual cash flows discount rate	7.00%	7.00%	7.00%
Escrow earn rate	0.80%	0.90%	1.00%
Servicing cost	$ 150,000,000	$ 150,000,000	$ 150,000,000
Loan assumption rate	0.00%	0.00%	0.00%
Percentage late	0.00%	0.00%	0.00%
Commercial Mortgage Backed Securities [Member] \| Maximum [Member]
Servicing Assets at Fair Value [Line Items]
Prepayment speed	20.40%	16.30%	17.20%
Expected defaults	3.00%	3.00%	3.00%
Residual cash flows discount rate	12.10%	15.00%	15.00%
Escrow earn rate	2.60%	3.50%	3.50%
Servicing cost	$ 2,700,000,000	$ 2,719,000,000	$ 2,700,000,000
Loan assumption rate	3.00%	3.00%	3.00%
Percentage late	2.00%	2.00%	2.00%
Commercial Mortgage Backed Securities [Member] \| Weighted Average [Member]
Servicing Assets at Fair Value [Line Items]
Prepayment speed	3.70%	4.90%	4.60%
Expected defaults	1.50%	1.70%	1.70%
Residual cash flows discount rate	7.90%	7.80%	7.80%
Escrow earn rate	2.00%	2.30%	2.30%
Servicing cost	$ 1,163,000,000	$ 1,151,000,000	$ 1,215,000,000
Loan assumption rate	1.18%	1.37%	1.34%
Percentage late	0.35%	0.33%	0.33%
Residential Mortgage Backed Securities [Member] \| Minimum [Member]
Servicing Assets at Fair Value [Line Items]
Prepayment speed	9.36%
Expected defaults	8.50%
Servicing cost	$ 76,000,000
Residential Mortgage Backed Securities [Member] \| Maximum [Member]
Servicing Assets at Fair Value [Line Items]
Prepayment speed	18.39%
Expected defaults	11.00%
Servicing cost	$ 3,335,000,000
Residential Mortgage Backed Securities [Member] \| Weighted Average [Member]
Servicing Assets at Fair Value [Line Items]
Prepayment speed	11.81%
Expected defaults	8.55%
Servicing cost	$ 82,170,000

Interest rate used to account for mortgage loan at fair value upon assumption.

No definition available.

Name:	key_InterestRateUsedToAccountForMortgageLoanAtFairValueUponAssumption
Namespace Prefix:	key_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Mortgage servicing assets, assumptions used to estimate fair value, percentage late.

+ References

No definition available.

+ Details

Name:	key_MortgageServicingAssetsAssumptionsUsedToEstimateFairValuePercentageLate
Namespace Prefix:	key_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Mortgage servicing assets interest rate for mortgage funds.

+ References

No definition available.

+ Details

Name:	key_MortgageServicingAssetsInterestRateForMortgageFunds
Namespace Prefix:	key_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Mortgage servicing assets servicing cost.

+ References

No definition available.

+ Details

Name:	key_MortgageServicingAssetsServicingCost
Namespace Prefix:	key_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Discount rate used to estimate the fair value of servicing assets and servicing liabilities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122596-111746

+ Details

Name:	us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of contractually collectible principal associated with a financial asset expected not to be collected due to default by the borrower.

+ References

+ Details

Name:	us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueExpectedCreditLosses
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Estimated rate of prepayments of principal on servicing assets and servicing liabilities.

+ References

+ Details

Name:	us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValuePrepaymentSpeed
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

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

- Details

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

- Details

Name:	us-gaap_StatementEquityComponentsAxis=key_DiscountedCashFlowMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_WeightedAverageMember
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: