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, 2014	Mar. 31, 2013	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	$ 34	$ 34	$ 42
Collectively Evaluated for Impairment Allowance	799	859	805
Purchased Credit Impaired Allowance	1		1
Total Loans	55,445	52,574	54,457
Individually Evaluated for Impairment, Outstanding	304	445	358
Collectively Evaluated for Impairment, Outstanding	55,125	52,107	54,083
Purchased Credit Impaired, Outstanding	16	22	16
Individually Evaluated for Impairment Allowance, Discontinued Operations	1	1	1
Collectively Evaluated for Impairment Allowance, Discontinued Operations	33	48	38
Purchase Credit Impairment Allowance, Discontinued Operations
Loans Outstanding, Discontinued Operations	4,382	5,086	4,497
Individually Evaluated for Impairment Outstanding, Discontinued Operations	15	4	13
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	4,367	5,082	4,484
Purchase Credit Impairment Outstanding, Discontinued Operations
Individually Evaluated for Impairment Allowance, Total ALLL - Including Discontinued Operations	35	35	43
Collectively Evaluated for Impairment Allowance, Total ALLL - Including Discontinued Operations	832	907	843
Purchased Credit Impaired Allowance, Total ALLL - Including Discontinued Operations	1		1
Loans Outstanding, Total ALLL - Including Discontinued Operations	59,827	57,660	58,954
Individually Evaluated for Impairment Outstanding, Total ALLL - Including Discontinued Operations	319	449	371
Collectively Evaluated for Impairment Outstanding, Total ALLL - Including Discontinued Operations	59,492	57,189	58,567
Purchased Credit Impaired Outstanding, Total ALLL - Including Discontinued Operations	16	22	16
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	18	14	11
Collectively Evaluated for Impairment Allowance	73	110	84
Purchased Credit Impaired Allowance
Total Loans	10,596	10,210	10,674
Individually Evaluated for Impairment, Outstanding	118	102	114
Collectively Evaluated for Impairment, Outstanding	10,476	10,106	10,559
Purchased Credit Impaired, Outstanding	2	2	1
Credit Cards [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	1	1	1
Collectively Evaluated for Impairment Allowance	31	31	33
Purchased Credit Impaired Allowance
Total Loans	698	693	722
Individually Evaluated for Impairment, Outstanding	4	3	5
Collectively Evaluated for Impairment, Outstanding	694	690	717
Purchased Credit Impaired, Outstanding
Commercial, Financial and Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	2	8	8
Collectively Evaluated for Impairment Allowance	371	330	354
Purchased Credit Impaired Allowance
Total Loans	26,224	23,412	24,963
Individually Evaluated for Impairment, Outstanding	40	108	50
Collectively Evaluated for Impairment, Outstanding	26,184	23,303	24,913
Purchased Credit Impaired, Outstanding		1
Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	1	3	2
Collectively Evaluated for Impairment Allowance	160	190	163
Purchased Credit Impaired Allowance
Total Loans	7,877	7,544	7,720
Individually Evaluated for Impairment, Outstanding	25	96	27
Collectively Evaluated for Impairment, Outstanding	7,851	7,446	7,692
Purchased Credit Impaired, Outstanding	1	2	1
Commercial Real Estate: Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance
Collectively Evaluated for Impairment Allowance	37	35	32
Purchased Credit Impaired Allowance
Total Loans	1,007	1,057	1,093
Individually Evaluated for Impairment, Outstanding	6	48	50
Collectively Evaluated for Impairment, Outstanding	1,001	1,009	1,043
Purchased Credit Impaired, Outstanding
Commercial Real Estate Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	1	3	2
Collectively Evaluated for Impairment Allowance	197	225	195
Purchased Credit Impaired Allowance
Total Loans	8,884	8,601	8,813
Individually Evaluated for Impairment, Outstanding	31	144	77
Collectively Evaluated for Impairment, Outstanding	8,852	8,455	8,735
Purchased Credit Impaired, Outstanding	1	2	1
Commercial Lease Financing [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance
Collectively Evaluated for Impairment Allowance	62	62	62
Purchased Credit Impaired Allowance
Total Loans	4,396	4,796	4,551
Individually Evaluated for Impairment, Outstanding
Collectively Evaluated for Impairment, Outstanding	4,396	4,796	4,551
Purchased Credit Impaired, Outstanding
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	3	11	10
Collectively Evaluated for Impairment Allowance	630	617	611
Purchased Credit Impaired Allowance
Total Loans	39,504	36,809	38,327
Individually Evaluated for Impairment, Outstanding	71	252	127
Collectively Evaluated for Impairment, Outstanding	39,432	36,554	38,199
Purchased Credit Impaired, Outstanding	1	3	1
Real Estate - Residential Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	4	1	9
Collectively Evaluated for Impairment Allowance	22	33	27
Purchased Credit Impaired Allowance	1		1
Total Loans	2,183	2,176	2,187
Individually Evaluated for Impairment, Outstanding	54	33	56
Collectively Evaluated for Impairment, Outstanding	2,116	2,127	2,117
Purchased Credit Impaired, Outstanding	13	16	14
Home Equity: Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	16	14	10
Collectively Evaluated for Impairment Allowance	64	92	74
Purchased Credit Impaired Allowance
Total Loans	10,281	9,809	10,340
Individually Evaluated for Impairment, Outstanding	105	90	102
Collectively Evaluated for Impairment, Outstanding	10,174	9,717	10,237
Purchased Credit Impaired, Outstanding	2	2	1
Home Equity: Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	2		1
Collectively Evaluated for Impairment Allowance	9	18	10
Purchased Credit Impaired Allowance
Total Loans	315	401	334
Individually Evaluated for Impairment, Outstanding	13	12	12
Collectively Evaluated for Impairment, Outstanding	302	389	322
Purchased Credit Impaired, Outstanding
Consumer Other - Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance		1	1
Collectively Evaluated for Impairment Allowance	25	32	28
Purchased Credit Impaired Allowance
Total Loans	1,436	1,353	1,449
Individually Evaluated for Impairment, Outstanding	4	4	3
Collectively Evaluated for Impairment, Outstanding	1,432	1,348	1,446
Purchased Credit Impaired, Outstanding		1
Consumer Marine [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	7	5	10
Collectively Evaluated for Impairment Allowance	16	33	19
Purchased Credit Impaired Allowance
Total Loans	965	1,254	1,028
Individually Evaluated for Impairment, Outstanding	52	50	52
Collectively Evaluated for Impairment, Outstanding	913	1,204	976
Purchased Credit Impaired, Outstanding
Consumer Others [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	1	1
Collectively Evaluated for Impairment Allowance	2	3	3
Purchased Credit Impaired Allowance
Total Loans	63	79	70
Individually Evaluated for Impairment, Outstanding	1	1	1
Collectively Evaluated for Impairment, Outstanding	62	78	69
Purchased Credit Impaired, Outstanding
Consumer Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	8	6	10
Collectively Evaluated for Impairment Allowance	18	36	22
Purchased Credit Impaired Allowance
Total Loans	1,028	1,333	1,098
Individually Evaluated for Impairment, Outstanding	53	51	53
Collectively Evaluated for Impairment, Outstanding	975	1,282	1,045
Purchased Credit Impaired, Outstanding
Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	31	23	32
Collectively Evaluated for Impairment Allowance	169	242	194
Purchased Credit Impaired Allowance	1		1
Total Loans	15,941	15,765	16,130
Individually Evaluated for Impairment, Outstanding	233	193	231
Collectively Evaluated for Impairment, Outstanding	15,693	15,553	15,884
Purchased Credit Impaired, Outstanding	$ 15	$ 19	$ 15