Asset Quality - Allowance for Loan and Lease Losses and Corresponding Loan Balances (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	$ 49	$ 34	$ 40
Collectively Evaluated for Impairment Allowance	744	799	753
Purchased Credit Impaired Allowance	1key_AllowanceForCreditImpairment	1key_AllowanceForCreditImpairment	1key_AllowanceForCreditImpairment
Total Loans	57,953	55,445	57,381
Individually Evaluated for Impairment Outstanding, Discontinued Operations	338	304	302
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	57,603	55,125	57,066
Purchase Credit Impairment Outstanding, Discontinued Operations	12	16	13
Individually Evaluated for Impairment Allowance, Total ALLL - Including Discontinued Operations	50	35	41
Collectively Evaluated for Impairment Allowance, Total ALLL - Including Discontinued Operations	768	832	781
Purchased Credit Impaired Allowance, Total ALLL - Including Discontinued Operations	1	1	1
Loans Outstanding, Total ALLL - Including Discontinued Operations	60,172	59,827	59,676
Individually Evaluated for Impairment Outstanding, Total ALLL - Including Discontinued Operations	356	319	319
Collectively Evaluated for Impairment Outstanding, Total ALLL - Including Discontinued Operations	59,804	59,492	59,344
Purchased Credit Impaired Outstanding, Total ALLL - Including Discontinued Operations	12	16	13
Discontinued Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total Loans	2,032	2,318	2,104
Individually Evaluated for Impairment Outstanding, Discontinued Operations	18	15	17
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	2,201	4,367	2,278
Individually Evaluated for Impairment Allowance, Discontinued Operations	1	1	1
Collectively Evaluated for Impairment Allowance, Discontinued Operations	24	33	28
Purchase Credit Impairment Allowance, Discontinued Operations	0	0	0
Loans Outstanding, Discontinued Operations	2,219	4,382	2,295
Commercial, Financial and Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	20	2	9
Collectively Evaluated for Impairment Allowance	385	371	382
Total Loans	28,783	26,224	27,982
Individually Evaluated for Impairment Outstanding, Discontinued Operations	82	40	43
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	28,701	26,184	27,939
Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	2	1	2
Collectively Evaluated for Impairment Allowance	146	160	146
Total Loans	8,162	7,877	8,047
Individually Evaluated for Impairment Outstanding, Discontinued Operations	20	25	21
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	8,142	7,851	8,025
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	28	37	27
Total Loans	1,142	1,007	1,100
Individually Evaluated for Impairment Outstanding, Discontinued Operations	7	6	8
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	1,135	1,001	1,092
Commercial Real Estate Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	2	1	3
Collectively Evaluated for Impairment Allowance	174	197	173
Total Loans	9,304	8,884	9,147
Individually Evaluated for Impairment Outstanding, Discontinued Operations	27	31	29
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	9,277	8,852	9,117
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	55	62	56
Total Loans	4,064	4,396	4,252
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	4,064	4,396	4,252
Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	22	3	12
Collectively Evaluated for Impairment Allowance	614	630	611
Total Loans	42,151	39,504	41,381
Individually Evaluated for Impairment Outstanding, Discontinued Operations	109	71	72
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	42,042	39,432	41,308
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	5	4	5
Collectively Evaluated for Impairment Allowance	15	22	17
Purchased Credit Impaired Allowance	1	1	1
Total Loans	2,231	2,183	2,225
Individually Evaluated for Impairment Outstanding, Discontinued Operations	55	54	55
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	2,165	2,116	2,159
Purchase Credit Impairment Outstanding, Discontinued Operations	11	13	11
Home Equity: Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	16	16	16
Collectively Evaluated for Impairment Allowance	42	64	50
Total Loans	10,270	10,281	10,366
Individually Evaluated for Impairment Outstanding, Discontinued Operations	111	105	108
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	10,158	10,174	10,257
Purchase Credit Impairment Outstanding, Discontinued Operations	1	2	1
Home Equity: Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	2	2	2
Collectively Evaluated for Impairment Allowance	3	9	3
Total Loans	253	315	267
Individually Evaluated for Impairment Outstanding, Discontinued Operations	12	13	12
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	241	302	255
Home Equity Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	18	18	18
Collectively Evaluated for Impairment Allowance	45	73	53
Total Loans	10,523	10,596	10,633
Individually Evaluated for Impairment Outstanding, Discontinued Operations	123	118	120
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	10,399	10,476	10,512
Purchase Credit Impairment Outstanding, Discontinued Operations	1	2	1
Consumer Other - Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively Evaluated for Impairment Allowance	21	25	22
Total Loans	1,547	1,436	1,560
Individually Evaluated for Impairment Outstanding, Discontinued Operations	3	4	4
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	1,544	1,432	1,556
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	32	31	33
Total Loans	727	698	754
Individually Evaluated for Impairment Outstanding, Discontinued Operations	4	4	4
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	723	694	750
Consumer Marine [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	4	7	5
Collectively Evaluated for Impairment Allowance	16	16	16
Total Loans	730	965	779
Individually Evaluated for Impairment Outstanding, Discontinued Operations	42	52	45
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	688	913	734
Consumer Others [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance		1
Collectively Evaluated for Impairment Allowance	1	2	1
Total Loans	44	63	49
Individually Evaluated for Impairment Outstanding, Discontinued Operations	2	1	2
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	42	62	47
Consumer Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	4	8	5
Collectively Evaluated for Impairment Allowance	17	18	17
Total Loans	774	1,028	828
Individually Evaluated for Impairment Outstanding, Discontinued Operations	44	53	47
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	730	975	781
Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	27	31	28
Collectively Evaluated for Impairment Allowance	130	169	142
Purchased Credit Impaired Allowance	1	1	1
Total Loans	15,802	15,941	16,000
Individually Evaluated for Impairment Outstanding, Discontinued Operations	229	233	230
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	15,561	15,693	15,758
Purchase Credit Impairment Outstanding, Discontinued Operations	$ 12	$ 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 allowance for credit losses collectively evaluated for impairment 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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Purchased credit impaired loans outstanding principal balance.

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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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The balance of financing receivables that were individually 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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