Asset Quality - Past Due Loans Including Current Loans (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 59,349	$ 56,619	$ 55,328
30-59 Days Past Due	194	147	235
60-89 Days Past Due	77	88	106
90 and Greater Days Past Due	53	96	71
Nonperforming Loans	400	418	401
Total Past Due and Nonperforming Loans	724	749	813
Purchased Credit Impaired	12	13	14
Loans Outstanding	60,085	57,381	56,155
Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	30,901	27,858	26,534
30-59 Days Past Due	58	19	50
60-89 Days Past Due	30	14	34
90 and Greater Days Past Due	17	32	18
Nonperforming Loans	89	59	47
Total Past Due and Nonperforming Loans	194	124	149
Loans Outstanding	31,095	27,982	26,683
Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	8,127	7,981	8,201
30-59 Days Past Due	18	6	17
60-89 Days Past Due	7	10	7
90 and Greater Days Past Due	5	16	9
Nonperforming Loans	23	34	41
Total Past Due and Nonperforming Loans	53	66	74
Purchased Credit Impaired			1
Loans Outstanding	8,180	8,047	8,276
Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,060	1,084	1,017
30-59 Days Past Due	1	2	3
60-89 Days Past Due			2
90 and Greater Days Past Due		1
Nonperforming Loans	9	13	14
Total Past Due and Nonperforming Loans	10	16	19
Loans Outstanding	1,070	1,100	1,036
Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	9,187	9,065	9,218
30-59 Days Past Due	19	8	20
60-89 Days Past Due	7	10	9
90 and Greater Days Past Due	5	17	9
Nonperforming Loans	32	47	55
Total Past Due and Nonperforming Loans	63	82	93
Purchased Credit Impaired			1
Loans Outstanding	9,250	9,147	9,312
Commercial Lease Financing [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	3,875	4,172	4,017
30-59 Days Past Due	29	30	74
60-89 Days Past Due	3	21	24
90 and Greater Days Past Due	1	11	6
Nonperforming Loans	21	18	14
Total Past Due and Nonperforming Loans	54	80	118
Loans Outstanding	3,929	4,252	4,135
Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	43,963	41,095	39,769
30-59 Days Past Due	106	57	144
60-89 Days Past Due	40	45	67
90 and Greater Days Past Due	23	60	33
Nonperforming Loans	142	124	116
Total Past Due and Nonperforming Loans	311	286	360
Purchased Credit Impaired			1
Loans Outstanding	44,274	41,381	40,130
Real Estate - Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,171	2,111	2,091
30-59 Days Past Due	11	12	17
60-89 Days Past Due	4	7	7
90 and Greater Days Past Due	3	4	5
Nonperforming Loans	67	79	81
Total Past Due and Nonperforming Loans	85	102	110
Purchased Credit Impaired	11	12	12
Loans Outstanding	2,267	2,225	2,213
Home Equity: Key Community Bank [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	10,027	10,098	10,124
30-59 Days Past Due	49	46	46
60-89 Days Past Due	20	22	19
90 and Greater Days Past Due	11	14	16
Nonperforming Loans	174	185	174
Total Past Due and Nonperforming Loans	254	267	255
Purchased Credit Impaired	1	1	1
Loans Outstanding	10,282	10,366	10,380
Home Equity: Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	208	249	266
30-59 Days Past Due	4	5	4
60-89 Days Past Due	2	2	2
90 and Greater Days Past Due	1	1	1
Nonperforming Loans	7	10	10
Total Past Due and Nonperforming Loans	14	18	17
Loans Outstanding	222	267	283
Home Equity Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	10,235	10,347	10,390
30-59 Days Past Due	53	51	50
60-89 Days Past Due	22	24	21
90 and Greater Days Past Due	12	15	17
Nonperforming Loans	181	195	184
Total Past Due and Nonperforming Loans	268	285	272
Purchased Credit Impaired	1	1	1
Loans Outstanding	10,504	10,633	10,663
Consumer Other - Key Community Bank [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,595	1,541	1,528
30-59 Days Past Due	7	9	7
60-89 Days Past Due	4	3	3
90 and Greater Days Past Due	5	5	6
Nonperforming Loans	1	2	2
Total Past Due and Nonperforming Loans	17	19	18
Loans Outstanding	1,612	1,560	1,546
Consumer Credit Card Financing Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	750	733	705
30-59 Days Past Due	6	6	5
60-89 Days Past Due	4	4	4
90 and Greater Days Past Due	8	9	9
Nonperforming Loans	2	2	1
Total Past Due and Nonperforming Loans	20	21	19
Loans Outstanding	770	754	724
Consumer Marine [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	601	746	796
30-59 Days Past Due	10	11	11
60-89 Days Past Due	2	5	4
90 and Greater Days Past Due	1	2	1
Nonperforming Loans	6	15	16
Total Past Due and Nonperforming Loans	19	33	32
Loans Outstanding	620	779	828
Consumer Others [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	34	46	49
30-59 Days Past Due	1	1	1
60-89 Days Past Due	1
90 and Greater Days Past Due	1	1
Nonperforming Loans	1	1	1
Total Past Due and Nonperforming Loans	4	3	2
Loans Outstanding	38	49	51
Consumer Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	635	792	845
30-59 Days Past Due	11	12	12
60-89 Days Past Due	3	5	4
90 and Greater Days Past Due	2	3	1
Nonperforming Loans	7	16	17
Total Past Due and Nonperforming Loans	23	36	34
Loans Outstanding	658	828	879
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	15,386	15,524	15,559
30-59 Days Past Due	88	90	91
60-89 Days Past Due	37	43	39
90 and Greater Days Past Due	30	36	38
Nonperforming Loans	258	294	285
Total Past Due and Nonperforming Loans	413	463	453
Purchased Credit Impaired	12	13	13
Loans Outstanding	$ 15,811	$ 16,000	$ 16,025