Asset Quality - Allowance for Loan and Lease Losses and Corresponding Loan Balances (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2013	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	$ 42	$ 40
Collectively Evaluated for Impairment Allowance	805	753
Purchased Credit Impaired Allowance	1key_AllowanceForCreditImpairment	1key_AllowanceForCreditImpairment
Total Loans	54,457	57,381
Individually Evaluated for Impairment Outstanding, Discontinued Operations	358	302
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	54,083.0	57,066.0
Purchase Credit Impairment Outstanding, Discontinued Operations	16	13
Individually Evaluated for Impairment Allowance, Discontinued Operations	1
Collectively Evaluated for Impairment Allowance, Discontinued Operations	38
Purchase Credit Impairment Allowance, Discontinued Operations	0
Loans Outstanding, Discontinued Operations	4,497key_LoansDiscontinuedOperations
Individually Evaluated for Impairment Allowance, Total ALLL - Including Discontinued Operations	43	41
Collectively Evaluated for Impairment Allowance, Total ALLL - Including Discontinued Operations	843	781
Purchased Credit Impaired Allowance, Total ALLL - Including Discontinued Operations	1	1
Loans Outstanding, Total ALLL - Including Discontinued Operations	58,954	59,676
Individually Evaluated for Impairment Outstanding, Total ALLL - Including Discontinued Operations	371	319
Collectively Evaluated for Impairment Outstanding, Total ALLL - Including Discontinued Operations	58,567	59,344
Purchased Credit Impaired Outstanding, Total ALLL - Including Discontinued Operations	16	13
Individually Evaluated for Impairment Outstanding, Discontinued Operations	13
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	4,484
Purchase Credit Impairment Outstanding, Discontinued Operations	0
Segment, Discontinued Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	2,390	2,104
Individually Evaluated for Impairment Outstanding, Discontinued Operations		17
Collectively Evaluated for Impairment Outstanding, Discontinued Operations		2,278.0
Individually Evaluated for Impairment Allowance, Discontinued Operations		1
Collectively Evaluated for Impairment Allowance, Discontinued Operations		28
Purchase Credit Impairment Allowance, Discontinued Operations		0
Loans Outstanding, Discontinued Operations		2,295
Commercial, Financial and Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	8	9
Collectively Evaluated for Impairment Allowance	354	382
Total Loans	24,963	27,982
Individually Evaluated for Impairment Outstanding, Discontinued Operations	50	43
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	24,913.0	27,939.0
Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	2	2
Collectively Evaluated for Impairment Allowance	163	146
Total Loans	7,720	8,047
Individually Evaluated for Impairment Outstanding, Discontinued Operations	27	21
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	7,692.0	8,025.0
Purchase Credit Impairment Outstanding, Discontinued Operations	1	1
Commercial Real Estate: Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance		1
Collectively Evaluated for Impairment Allowance	32	27
Total Loans	1,093	1,100
Individually Evaluated for Impairment Outstanding, Discontinued Operations	50	8
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	1,043.0	1,092.0
Commercial Real Estate Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	2	3
Collectively Evaluated for Impairment Allowance	195	173
Total Loans	8,813	9,147
Individually Evaluated for Impairment Outstanding, Discontinued Operations	77	29
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	8,735.0	9,117.0
Purchase Credit Impairment Outstanding, Discontinued Operations	1	1
Commercial Lease Financing [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively Evaluated for Impairment Allowance	62	56
Total Loans	4,551	4,252
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	4,551.0	4,252.0
Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	10	12
Collectively Evaluated for Impairment Allowance	611	611
Total Loans	38,327	41,381
Individually Evaluated for Impairment Outstanding, Discontinued Operations	127	72
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	38,199.0	41,308.0
Purchase Credit Impairment Outstanding, Discontinued Operations	1	1
Real Estate - Residential Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	9	5
Collectively Evaluated for Impairment Allowance	27	17
Purchased Credit Impaired Allowance	1	1
Total Loans	2,187	2,225
Individually Evaluated for Impairment Outstanding, Discontinued Operations	56	55
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	2,117.0	2,159.0
Purchase Credit Impairment Outstanding, Discontinued Operations	14	11
Home Equity: Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	10	16
Collectively Evaluated for Impairment Allowance	74	50
Total Loans	10,340	10,366
Individually Evaluated for Impairment Outstanding, Discontinued Operations	102	108
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	10,237.0	10,257.0
Purchase Credit Impairment Outstanding, Discontinued Operations	1	1
Home Equity: Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	1	2
Collectively Evaluated for Impairment Allowance	10	3
Total Loans	334	267
Individually Evaluated for Impairment Outstanding, Discontinued Operations	12	12
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	322.0	255.0
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	11	18
Collectively Evaluated for Impairment Allowance	84	53
Total Loans	10,674	10,633
Individually Evaluated for Impairment Outstanding, Discontinued Operations	114	120
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	10,559.0	10,512.0
Purchase Credit Impairment Outstanding, Discontinued Operations	1	1
Consumer Other - Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	1
Collectively Evaluated for Impairment Allowance	28	22
Total Loans	1,449	1,560
Individually Evaluated for Impairment Outstanding, Discontinued Operations	3	4
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	1,446.0	1,556.0
Consumer Credit Card Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	1
Collectively Evaluated for Impairment Allowance	33	33
Total Loans	722	754
Individually Evaluated for Impairment Outstanding, Discontinued Operations	5	4
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	717.0	750.0
Consumer Marine [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	10	5
Collectively Evaluated for Impairment Allowance	19	16
Total Loans	1,028	779
Individually Evaluated for Impairment Outstanding, Discontinued Operations	52	45
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	976.0	734.0
Consumer Others [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively Evaluated for Impairment Allowance	3	1
Total Loans	70	49
Individually Evaluated for Impairment Outstanding, Discontinued Operations	1	2
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	69.0	47.0
Consumer Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	10	5
Collectively Evaluated for Impairment Allowance	22	17
Total Loans	1,098	828
Individually Evaluated for Impairment Outstanding, Discontinued Operations	53	47
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	1,045.0	781.0
Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	32	28
Collectively Evaluated for Impairment Allowance	194	142
Purchased Credit Impaired Allowance	1	1
Total Loans	16,130	16,000
Individually Evaluated for Impairment Outstanding, Discontinued Operations	231	230
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	15,884.0	15,758.0
Purchase Credit Impairment Outstanding, Discontinued Operations	$ 15	$ 12

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Financing receivable allowance for credit losses collectively evaluated for impairment total ALLL including discontinued operations.

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Financing receivable collectively evaluated for impairment total all including discontinued operations.

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Loans total ALLL including discontinued operations.

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Loans discontinued operations.

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The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

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The balance of financing receivables that were collectively evaluated for impairment.

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Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

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