Asset Quality - Allowance for Loan and Lease Losses and Corresponding Loan Balances (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	$ 34	$ 35	$ 48
Collectively Evaluated for Impairment Allowance	842	852	840
Purchased Credit Impaired Allowance		1
Loans Outstanding	53,101	52,822	49,605
Individually Evaluated for Impairment, Outstanding	464	411	411
Collectively Evaluated for Impairment, Outstanding	52,618	52,388	49,194
Purchased Credit Impaired, Outstanding	19	23
Individually Evaluated for Impairment Allowance, Discontinued Operations	2
Collectively Evaluated for Impairment Allowance, Discontinued Operations	39	55	79
Loans Outstanding, Discontinued Operations	4,992	5,201	5,483
Individually Evaluated for Impairment Outstanding, Discontinued Operations	8	3
Collectively Evaluated for Impairment Allowance, Discontinued Operations	4,984	5,198	5,483
Individually Evaluated for Impairment Allowance, Total ALLL - Including Discontinued Operations	36	35	48
Collectively Evaluated for Impairment Allowance, Total ALLL - Including Discontinued Operations	881	907	919
Purchased Credit Impaired Allowance, Total ALLL - Including Discontinued Operations		1
Loans Outstanding, Total ALLL - Including Discontinued Operations	58,093	58,023	55,088
Individually Evaluated for Impairment Outstanding, Total ALLL - Including Discontinued Operations	472	414	411
Collectively Evaluated for Impairment Outstanding, Total ALLL - Including Discontinued Operations	57,602	57,586	54,677
Purchased Credit Impaired Outstanding, Total ALLL - Including Discontinued Operations	19	23
Commercial, Financial and Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	6	12	12
Collectively Evaluated for Impairment Allowance	346	314	292
Loans Outstanding	23,715	23,242	20,916
Individually Evaluated for Impairment, Outstanding	111	65	102
Collectively Evaluated for Impairment, Outstanding	23,604	23,176	20,814
Purchased Credit Impaired, Outstanding		1
Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	2	1	15
Collectively Evaluated for Impairment Allowance	180	198	235
Loans Outstanding	7,474	7,720	7,409
Individually Evaluated for Impairment, Outstanding	93	96	168
Collectively Evaluated for Impairment, Outstanding	7,379	7,621	7,241
Purchased Credit Impaired, Outstanding	2	3
Commercial Real Estate: Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance			3
Collectively Evaluated for Impairment Allowance	34	41	52
Loans Outstanding	1,060	1,003	1,172
Individually Evaluated for Impairment, Outstanding	52	48	55
Collectively Evaluated for Impairment, Outstanding	1,008	955	1,117
Commercial Real Estate Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	2	1	18
Collectively Evaluated for Impairment Allowance	214	239	287
Loans Outstanding	8,534	8,723	8,581
Individually Evaluated for Impairment, Outstanding	145	144	223
Collectively Evaluated for Impairment, Outstanding	8,387	8,576	8,358
Purchased Credit Impaired, Outstanding	2	3
Commercial Lease Financing [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively Evaluated for Impairment Allowance	61	55	68
Loans Outstanding	4,774	4,915	5,106
Collectively Evaluated for Impairment, Outstanding	4,774	4,915	5,106
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	8	13	30
Collectively Evaluated for Impairment Allowance	621	608	647
Loans Outstanding	37,023	36,880	34,603
Individually Evaluated for Impairment, Outstanding	256	209	325
Collectively Evaluated for Impairment, Outstanding	36,765	36,667	34,278
Purchased Credit Impaired, Outstanding	2	4
Real Estate - Residential Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	5	1	2
Collectively Evaluated for Impairment Allowance	28	29	24
Purchased Credit Impaired Allowance		1
Loans Outstanding	2,176	2,174	2,016
Individually Evaluated for Impairment, Outstanding	35	38	17
Collectively Evaluated for Impairment, Outstanding	2,126	2,120	1,999
Purchased Credit Impaired, Outstanding	15	16
Home Equity: Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	9	11	3
Collectively Evaluated for Impairment Allowance	85	94	77
Loans Outstanding	10,173	9,816	9,601
Individually Evaluated for Impairment, Outstanding	99	87	11
Collectively Evaluated for Impairment, Outstanding	10,072	9,726	9,590
Purchased Credit Impaired, Outstanding	2	3
Home Equity: Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	1	1	1
Collectively Evaluated for Impairment Allowance	15	24	23
Loans Outstanding	375	423	479
Individually Evaluated for Impairment, Outstanding	13	12	6
Collectively Evaluated for Impairment, Outstanding	362	411	473
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	10	12	4
Collectively Evaluated for Impairment Allowance	100	118	100
Loans Outstanding	10,548	10,239	10,080
Individually Evaluated for Impairment, Outstanding	112	99	17
Collectively Evaluated for Impairment, Outstanding	10,434	10,137	10,063
Purchased Credit Impaired, Outstanding	2	3
Consumer Other - Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance		2	1
Collectively Evaluated for Impairment Allowance	33	36	33
Loans Outstanding	1,424	1,349	1,263
Individually Evaluated for Impairment, Outstanding	3	2	2
Collectively Evaluated for Impairment, Outstanding	1,421	1,347	1,261
Credit Cards [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively Evaluated for Impairment Allowance	33	26
Loans Outstanding	701	729
Individually Evaluated for Impairment, Outstanding	4	2
Collectively Evaluated for Impairment, Outstanding	697	727
Consumer Marine [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	10	7	11
Collectively Evaluated for Impairment Allowance	25	32	33
Loans Outstanding	1,160	1,358	1,542
Individually Evaluated for Impairment, Outstanding	53	60	50
Collectively Evaluated for Impairment, Outstanding	1,107	1,298	1,492
Consumer Others [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	1
Collectively Evaluated for Impairment Allowance	2	3	3
Loans Outstanding	69	93	101
Individually Evaluated for Impairment, Outstanding	1	1
Collectively Evaluated for Impairment, Outstanding	68	92	101
Consumer Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	11	7	11
Collectively Evaluated for Impairment Allowance	27	35	36
Loans Outstanding	1,229	1,451	1,643
Individually Evaluated for Impairment, Outstanding	54	61	50
Collectively Evaluated for Impairment, Outstanding	1,175	1,390	1,593
Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	26	22	18
Collectively Evaluated for Impairment Allowance	221	244	193
Purchased Credit Impaired Allowance		1
Loans Outstanding	16,078	15,942	15,002
Individually Evaluated for Impairment, Outstanding	208	202	86
Collectively Evaluated for Impairment, Outstanding	15,853	15,721	14,916
Purchased Credit Impaired, Outstanding	$ 17	$ 19