Asset Quality - Allowance for Loan and Lease Losses and Corresponding Loan Balances (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	$ 47	$ 39	$ 35
Collectively Evaluated for Impairment Allowance	820	849	852
Purchased Credit Impaired Allowance	1		1
Total loans	53,597	51,419	52,822
Individually Evaluated for Impairment, Outstanding	411	428	411
Collectively Evaluated for Impairment, Outstanding	53,168	50,966	52,388
Purchased Credit Impaired, Outstanding	18	25	23
Individually Evaluated for Impairment Allowance, Discontinued Operations	1
Collectively Evaluated for Impairment Allowance, Discontinued Operations	37	65	55
Purchase Credit Impairment Allowance, Discontinued Operations
Loans Outstanding, Discontinued Operations	4,738	5,328	5,201
Individually Evaluated for Impairment Outstanding, Discontinued Operations	11	3	3
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	4,727	5,325	5,198
Purchase Credit Impairment Outstanding, Discontinued Operations
Individually Evaluated for Impairment Allowance, Total ALLL - Including Discontinued Operations	48	39	35
Collectively Evaluated for Impairment Allowance, Total ALLL - Including Discontinued Operations	857	914	907
Purchased Credit Impaired Allowance, Total ALLL - Including Discontinued Operations	1		1
Loans Outstanding, Total ALLL - Including Discontinued Operations	58,335	56,747	58,023
Individually Evaluated for Impairment Outstanding, Total ALLL - Including Discontinued Operations	422	431	414
Collectively Evaluated for Impairment Outstanding, Total ALLL - Including Discontinued Operations	57,895	56,291	57,586
Purchased Credit Impaired Outstanding, Total ALLL - Including Discontinued Operations	18	25	23
Commercial, Financial and Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	17	12	12
Collectively Evaluated for Impairment Allowance	353	306	314
Purchased Credit Impaired Allowance
Total loans	24,317	21,979	23,242
Individually Evaluated for Impairment, Outstanding	108	92	65
Collectively Evaluated for Impairment, Outstanding	24,209	21,886	23,176
Purchased Credit Impaired, Outstanding		1	1
Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	1	7	1
Collectively Evaluated for Impairment Allowance	171	214	198
Purchased Credit Impaired Allowance
Total loans	7,544	7,529	7,720
Individually Evaluated for Impairment, Outstanding	46	138	96
Collectively Evaluated for Impairment, Outstanding	7,496	7,388	7,621
Purchased Credit Impaired, Outstanding	2	3	3
Commercial Real Estate: Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance
Collectively Evaluated for Impairment Allowance	36	47	41
Purchased Credit Impaired Allowance
Total loans	1,058	1,067	1,003
Individually Evaluated for Impairment, Outstanding	44	42	48
Collectively Evaluated for Impairment, Outstanding	1,014	1,025	955
Commercial Real Estate Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	1	7	1
Collectively Evaluated for Impairment Allowance	207	261	239
Purchased Credit Impaired Allowance
Total loans	8,602	8,596	8,723
Individually Evaluated for Impairment, Outstanding	90	180	144
Collectively Evaluated for Impairment, Outstanding	8,510	8,413	8,576
Purchased Credit Impaired, Outstanding	2	3	3
Commercial Lease Financing [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance
Collectively Evaluated for Impairment Allowance	64	66	55
Purchased Credit Impaired Allowance
Total loans	4,550	4,960	4,915
Individually Evaluated for Impairment, Outstanding
Collectively Evaluated for Impairment, Outstanding	4,550	4,960	4,915
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	18	19	13
Collectively Evaluated for Impairment Allowance	624	633	608
Purchased Credit Impaired Allowance
Total loans	37,469	35,535	36,880
Individually Evaluated for Impairment, Outstanding	198	272	209
Collectively Evaluated for Impairment, Outstanding	37,269	35,259	36,667
Purchased Credit Impaired, Outstanding	2	4	4
Real Estate - Residential Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	6	1	1
Collectively Evaluated for Impairment Allowance	28	24	29
Purchased Credit Impaired Allowance	1		1
Total loans	2,198	2,138	2,174
Individually Evaluated for Impairment, Outstanding	36	18	38
Collectively Evaluated for Impairment, Outstanding	2,148	2,103	2,120
Purchased Credit Impaired, Outstanding	14	17	16
Home Equity: Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	10	10	11
Collectively Evaluated for Impairment Allowance	72	79	94
Purchased Credit Impaired Allowance
Total loans	10,285	9,768	9,816
Individually Evaluated for Impairment, Outstanding	102	65	87
Collectively Evaluated for Impairment, Outstanding	10,181	9,700	9,726
Purchased Credit Impaired, Outstanding	2	3	3
Home Equity: Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	2	1	1
Collectively Evaluated for Impairment Allowance	12	23	24
Purchased Credit Impaired Allowance
Total loans	353	409	423
Individually Evaluated for Impairment, Outstanding	13	10	12
Collectively Evaluated for Impairment, Outstanding	340	399	411
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	12	11	12
Collectively Evaluated for Impairment Allowance	84	102	118
Purchased Credit Impaired Allowance
Total loans	10,638	10,177	10,239
Individually Evaluated for Impairment, Outstanding	115	75	99
Collectively Evaluated for Impairment, Outstanding	10,521	10,099	10,137
Purchased Credit Impaired, Outstanding	2	3	3
Consumer Other - Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance		1	2
Collectively Evaluated for Impairment Allowance	27	31	36
Purchased Credit Impaired Allowance
Total loans	1,440	1,313	1,349
Individually Evaluated for Impairment, Outstanding	3	2	2
Collectively Evaluated for Impairment, Outstanding	1,437	1,310	1,347
Purchased Credit Impaired, Outstanding		1
Credit Cards [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	1
Collectively Evaluated for Impairment Allowance	33	25	26
Purchased Credit Impaired Allowance
Total loans	698	710	729
Individually Evaluated for Impairment, Outstanding	6		2
Collectively Evaluated for Impairment, Outstanding	692	710	727
Consumer Marine [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	10	7	7
Collectively Evaluated for Impairment Allowance	21	30	32
Purchased Credit Impaired Allowance
Total loans	1,083	1,448	1,358
Individually Evaluated for Impairment, Outstanding	52	60	60
Collectively Evaluated for Impairment, Outstanding	1,031	1,388	1,298
Consumer Others [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance
Collectively Evaluated for Impairment Allowance	3	4	3
Purchased Credit Impaired Allowance
Total loans	71	98	93
Individually Evaluated for Impairment, Outstanding	1	1	1
Collectively Evaluated for Impairment, Outstanding	70	97	92
Consumer Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	10	7	7
Collectively Evaluated for Impairment Allowance	24	34	35
Purchased Credit Impaired Allowance
Total loans	1,154	1,546	1,451
Individually Evaluated for Impairment, Outstanding	53	61	61
Collectively Evaluated for Impairment, Outstanding	1,101	1,485	1,390
Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	29	20	22
Collectively Evaluated for Impairment Allowance	196	216	244
Purchased Credit Impaired Allowance	1		1
Total loans	16,128	15,884	15,942
Individually Evaluated for Impairment, Outstanding	213	156	202
Collectively Evaluated for Impairment, Outstanding	15,899	15,707	15,721
Purchased Credit Impaired, Outstanding	$ 16	$ 21	$ 19