Asset Quality - Allowance for Loan and Lease Losses and Corresponding Loan Balances (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans Outstanding	$ 60,085	$ 57,381	$ 56,155
Purchase Credit Impairment	12	13	14
Individually Evaluated for Impairment Allowance, Total ALLL - Including Discontinued Operations	38	41	39
Collectively Evaluated for Impairment Allowance, Total ALLL - Including Discontinued Operations	774	781	795
Purchased Credit Impaired Allowance, Total ALLL - Including Discontinued Operations	1	1	1
Loans Outstanding, Total ALLL - Including Discontinued Operations	61,976	59,676	58,547
Individually Evaluated for Impairment Outstanding, Total ALLL - Including Discontinued Operations	338	319	315
Collectively Evaluated for Impairment Outstanding, Total ALLL - Including Discontinued Operations	61,626	59,344	58,218
Purchased Credit Impaired Outstanding, Total ALLL - Including Discontinued Operations	12	13	14
Discontinued Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	2	1	1
Collectively Evaluated for Impairment Allowance	21	28	30
Purchase Credit Impairment Allowance	0
Loans Outstanding	1,891	2,104	2,174
Individually Evaluated for Impairment Outstanding	20	17	16
Collectively Evaluated for Impairment Outstanding	1,871	2,278	2,376
Purchase Credit Impairment	0
Loans Outstanding		2,295	2,392
Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	36	40	38
Collectively Evaluated for Impairment Allowance	753	753	765
Purchase Credit Impairment Allowance	1	1	1
Loans Outstanding	60,085	57,381	56,155
Individually Evaluated for Impairment Outstanding	318	302	299
Collectively Evaluated for Impairment Outstanding	59,755	57,066	55,842
Purchase Credit Impairment	12	13	14
Commercial, Financial and Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans Outstanding	31,095	27,982	26,683
Commercial, Financial and Agricultural [Member] | Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	9	9	7
Collectively Evaluated for Impairment Allowance	429	382	379
Purchase Credit Impairment Allowance	0
Loans Outstanding	31,095	27,982	26,683
Individually Evaluated for Impairment Outstanding	72	43	31
Collectively Evaluated for Impairment Outstanding	31,023	27,939	26,652
Purchase Credit Impairment	0
Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans Outstanding	8,180	8,047	8,276
Commercial Real Estate: Commercial Mortgage [Member] | Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	1	2	2
Collectively Evaluated for Impairment Allowance	138	146	157
Purchase Credit Impairment Allowance	0
Loans Outstanding	8,180	8,047	8,276
Individually Evaluated for Impairment Outstanding	15	21	29
Collectively Evaluated for Impairment Outstanding	8,165	8,025	8,246
Purchase Credit Impairment	0	1	1
Commercial Real Estate: Construction [Member] | Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	0	1
Collectively Evaluated for Impairment Allowance	25	27	28
Purchase Credit Impairment Allowance	0
Loans Outstanding	1,070	1,100	1,036
Individually Evaluated for Impairment Outstanding	5	8	10
Collectively Evaluated for Impairment Outstanding	1,065	1,092	1,026
Purchase Credit Impairment	0
Commercial Real Estate Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans Outstanding	9,250	9,147	9,312
Commercial Real Estate Loans [Member] | Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	1	3	2
Collectively Evaluated for Impairment Allowance	163	173	185
Purchase Credit Impairment Allowance	0
Loans Outstanding	9,250	9,147	9,312
Individually Evaluated for Impairment Outstanding	20	29	39
Collectively Evaluated for Impairment Outstanding	9,230	9,117	9,272
Purchase Credit Impairment	0	1	1
Commercial Lease Financing [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans Outstanding	3,929	4,252	4,135
Commercial Lease Financing [Member] | Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	0
Collectively Evaluated for Impairment Allowance	45	56	55
Purchase Credit Impairment Allowance	0
Loans Outstanding	3,929	4,252	4,135
Individually Evaluated for Impairment Outstanding	0
Collectively Evaluated for Impairment Outstanding	3,929	4,252	4,135
Purchase Credit Impairment	0
Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans Outstanding	44,274	41,381	40,130
Commercial Loans [Member] | Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	10	12	9
Collectively Evaluated for Impairment Allowance	637	611	619
Purchase Credit Impairment Allowance	0
Loans Outstanding	44,274	41,381	40,130
Individually Evaluated for Impairment Outstanding	92	72	70
Collectively Evaluated for Impairment Outstanding	44,182	41,308	40,059
Purchase Credit Impairment	0	1	1
Real Estate - Residential Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans Outstanding	2,267	2,225	2,213
Real Estate - Residential Mortgage [Member] | Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	5	5	4
Collectively Evaluated for Impairment Allowance	13	17	17
Purchase Credit Impairment Allowance	1	1	1
Loans Outstanding	2,267	2,225	2,213
Individually Evaluated for Impairment Outstanding	56	55	55
Collectively Evaluated for Impairment Outstanding	2,200	2,159	2,146
Purchase Credit Impairment	11	11	12
Home Equity: Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans Outstanding	10,282	10,366	10,380
Home Equity: Key Community Bank [Member] | Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	17	16	16
Collectively Evaluated for Impairment Allowance	37	50	55
Purchase Credit Impairment Allowance	0
Loans Outstanding	10,282	10,366	10,380
Individually Evaluated for Impairment Outstanding	113	108	105
Collectively Evaluated for Impairment Outstanding	10,168	10,257	10,274
Purchase Credit Impairment	1	1	1
Home Equity: Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans Outstanding	222	267	283
Home Equity: Other [Member] | Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	2	2	2
Collectively Evaluated for Impairment Allowance	2	3	4
Purchase Credit Impairment Allowance	0
Loans Outstanding	222	267	283
Individually Evaluated for Impairment Outstanding	11	12	12
Collectively Evaluated for Impairment Outstanding	211	255	271
Purchase Credit Impairment	0
Home Equity Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans Outstanding	10,504	10,633	10,663
Home Equity Loans [Member] | Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	19	18	18
Collectively Evaluated for Impairment Allowance	39	53	59
Purchase Credit Impairment Allowance	0
Loans Outstanding	10,504	10,633	10,663
Individually Evaluated for Impairment Outstanding	124	120	117
Collectively Evaluated for Impairment Outstanding	10,379	10,512	10,545
Purchase Credit Impairment	1	1	1
Consumer Other - Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans Outstanding	1,612	1,560	1,546
Consumer Other - Key Community Bank [Member] | Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	0
Collectively Evaluated for Impairment Allowance	20	22	24
Purchase Credit Impairment Allowance	0
Loans Outstanding	1,612	1,560	1,546
Individually Evaluated for Impairment Outstanding	3	4	4
Collectively Evaluated for Impairment Outstanding	1,609	1,556	1,542
Purchase Credit Impairment	0
Consumer Credit Card Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans Outstanding	770	754	724
Consumer Credit Card Financing Receivable [Member] | Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	0		1
Collectively Evaluated for Impairment Allowance	32	33	31
Purchase Credit Impairment Allowance	0
Loans Outstanding	770	754	724
Individually Evaluated for Impairment Outstanding	3	4	3
Collectively Evaluated for Impairment Outstanding	767	750	721
Purchase Credit Impairment	0
Consumer Marine [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans Outstanding	620	779	828
Consumer Marine [Member] | Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	2	5	5
Collectively Evaluated for Impairment Allowance	11	16	14
Purchase Credit Impairment Allowance	0
Loans Outstanding	620	779	828
Individually Evaluated for Impairment Outstanding	38	45	48
Collectively Evaluated for Impairment Outstanding	582	734	780
Purchase Credit Impairment	0
Consumer Others [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans Outstanding	38	49	51
Consumer Others [Member] | Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	0		1
Collectively Evaluated for Impairment Allowance	1	1	1
Purchase Credit Impairment Allowance	0
Loans Outstanding	38	49	51
Individually Evaluated for Impairment Outstanding	2	2	2
Collectively Evaluated for Impairment Outstanding	36	47	49
Purchase Credit Impairment	0
Consumer Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans Outstanding	658	828	879
Consumer Other [Member] | Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	2	5	6
Collectively Evaluated for Impairment Allowance	12	17	15
Purchase Credit Impairment Allowance	0
Loans Outstanding	658	828	879
Individually Evaluated for Impairment Outstanding	40	47	50
Collectively Evaluated for Impairment Outstanding	618	781	829
Purchase Credit Impairment	0
Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans Outstanding	15,811	16,000	16,025
Consumer Loans [Member] | Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually Evaluated for Impairment Allowance	26	28	29
Collectively Evaluated for Impairment Allowance	116	142	146
Purchase Credit Impairment Allowance	1	1	1
Loans Outstanding	15,811	16,000	16,025
Individually Evaluated for Impairment Outstanding	226	230	229
Collectively Evaluated for Impairment Outstanding	15,573	15,758	15,783
Purchase Credit Impairment	$ 12	$ 12	$ 13