Addition in restructured loans.

No definition available.

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- Definition

Aggregate carrying amount of largest nonperforming loans.

+ References

No definition available.

+ Details

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- Definition

Allowance for commercial loans fair value of underlying collateral payment due period.

+ References

No definition available.

+ Details

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- Definition

Allowance for consumer loans payment due period.

+ References

No definition available.

+ Details

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- Definition

Allowance for credit card loans payment due period.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Allowance for home equity and residential mortgage loans net realizable value payment due period.

+ References

No definition available.

+ Details

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- Definition

The estimated gross contractual amount receivable of purchase credit impaired loans.

+ References

No definition available.

+ Details

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- Definition

Minimum outstanding impaired commercial loans.

+ References

No definition available.

+ Details

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- Definition

Net reduction to interest income.

+ References

No definition available.

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- Definition

Non accretable difference acquisition.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

Number of branches sold.

+ References

No definition available.

+ Details

Name:	key_NumberOfBranchesSold
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Data Type:	xbrli:integerItemType
Balance Type:	na
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- Definition

Number of largest nonperforming loans.

+ References

No definition available.

+ Details

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- Definition

Percentage of carrying amount of our commercial nonperforming loans outstanding.

+ References

No definition available.

+ Details

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- Definition

Percentage of loans held for sale and other nonperforming assets.

+ References

No definition available.

+ Details

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- Definition

Percentage of total loans on nonperforming status.

+ References

No definition available.

+ Details

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- Definition

Percentage of nonperforming loans outstanding face value.

+ References

No definition available.

+ Details

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

- Definition

Total restructured loans.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

Restructured loans partially offset in payments and charge-offs .

+ References

No definition available.

+ Details

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

- Definition

Significant commitments to lend additional funds to borrowers with loans on nonperforming status.

+ References

No definition available.

+ Details

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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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Data Type:	xbrli:stringItemType
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Period Type:	duration

- Definition

For receivables acquired in a business combination, excluding certain loans and debt securities acquired in a transfer (as defined), this element represents the best estimate at the acquisition date of the contractual cash flows not expected to be collected, by major class of receivable, such as loans, direct finance leases (as defined), and any other class of receivables.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=73530397&loc=d3e4845-128472

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

- Definition

For receivables acquired in a business combination, excluding certain loans and debt securities acquired in a transfer (as defined), this element represents the fair value of the receivables acquired, by major class of receivable, such as loans, direct finance leases (as defined), and any other class of receivables.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=73530397&loc=d3e4845-128472

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- Definition

For receivables acquired in a business combination, excluding certain loans and debt securities acquired in a transfer (as defined), this element represents the gross contractual amounts receivable, by major class of receivable, such as loans, direct finance leases (as defined), and any other class of receivables.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=73530397&loc=d3e4845-128472

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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- Definition

The aggregate excess of a loan's cash flows expected to be collected over the investor's initial investment in the loan. This relates to loans with deteriorating of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

No definition available.

+ Details

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- Definition

Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953682-111524

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- Definition

Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default.

+ References

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- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5212-111524

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

- Definition

Amount of expense related to estimated loss from loan and lease transactions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953423-111524

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Asset Quality - Additional Information (Detail) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($) SecurityLoan	Jun. 30, 2016 USD ($) SecurityLoan	Jun. 30, 2017 USD ($) SecurityLoan	Jun. 30, 2016 USD ($) SecurityLoan	Dec. 31, 2016 USD ($)	Dec. 31, 2012 USD ($)	Sep. 09, 2016 Branch	Aug. 01, 2016 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of branches sold \| Branch							18
Estimated gross contractual amount receivable of PCI loans						$ 41.0
Estimated cash flows nonaccretable amount						11.0
Accretable yield						$ 5.0
Percentage of carrying amount of our commercial nonperforming loans outstanding	81.00%		81.00%
Percentage of nonperforming loans outstanding face value	83.00%		83.00%
Percentage of loans held for sale and other nonperforming assets	83.00%		83.00%
Number of largest nonperforming loans \| SecurityLoan	20		20
Aggregate carrying amount of largest nonperforming loans	$ 164.0		$ 164.0
Percentage of total loans on nonperforming status	32.00%		32.00%
Net reduction to interest income	$ 6.0	$ 6.0	$ 12.0	$ 12.0
Total restructured loans	333.0		333.0		$ 280.0
Addition in restructured loans	67.0		114.0		107.0
Restructured loans partially in payments and charge-offs	36.0		61.0		107.0
Financial receivable, modifications, subsequent default, recorded investment	2.0	2.0	2.0
Commitments outstanding to lend additional funds to borrowers	26.0		26.0		14.0
Minimum outstanding impaired commercial loans	2.5		$ 2.5
Number of days to designate commercial loans will be charged off in full or charged down to the fair value of the underlying collateral payment due period			180 days
Number of days to designate the charge-off policy for consumer loans taking effect, payment due period			120 days
Number of days to designate home equity and residential mortgage loans to get charged down to net realizable value payment due period			180 days
Number of days to designate the charge-off policy for credit card loans taking effect, payment due period			180 days
Provision (credit) for loan and lease losses	$ 66.0	$ 71.0	$ 139.0	$ 149.0
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans \| SecurityLoan	0	0	0	0
Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans \| SecurityLoan	52	41	55	91
Financial receivable, modifications, subsequent default, recorded investment				$ 4.0
Real Estate - Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financial receivable, modifications, subsequent default, recorded investment			$ 142.0		141.0
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income recognized on outstanding balances	$ 3.0	$ 1.0	6.0	6.0
Continuing Operations [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision (credit) for loan and lease losses	66.0	71.0	136.0	147.0
Continuing Operations [Member] \| Purchase Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision (credit) for loan and lease losses	1.0	1.0	4.0	1.0	$ 11.0
Continuing Operations [Member] \| Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision (credit) for loan and lease losses	55.0	61.0	97.0	107.0
Continuing Operations [Member] \| Consumer Portfolio Segment [Member] \| Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision (credit) for loan and lease losses	11.0	10.0	39.0	40.0
Continuing Operations [Member] \| Consumer Portfolio Segment [Member] \| Real Estate - Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision (credit) for loan and lease losses	$ (6.0)	$ (1.0)	$ (9.0)	$ 1.0
Purchase Credit Impaired Loans [Member] \| First Niagara Bank, N.A. [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required payments receivable								$ 22,500.0
Fair value of acquired receivables								22,000.0
Estimated cash flows not expected to be collected								500.0
Accretable yield								$ 172.0