Asset Quality - Past Due Loans Including Current Loans (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 57,177	$ 56,619	$ 54,624
30 - 59 Days Past Due	137	147	186
60 - 89 Days Past Due	79	88	81
90 and Greater Days Past Due	111	96	89
Nonperforming Loans	437	418	449
Total Past Due and Nonperforming Loans	764	749	805
Purchased Credit Impaired	12	13	16
Total Loans	57,953	57,381	55,445
Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	28,603	27,858	26,071
30 - 59 Days Past Due	36	19	67
60 - 89 Days Past Due	11	14	8
90 and Greater Days Past Due	35	32	18
Nonperforming Loans	98	59	60
Total Past Due and Nonperforming Loans	180	124	153
Total Loans	28,783	27,982	26,224
Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	8,080	7,981	7,806
30 - 59 Days Past Due	5	6	9
60 - 89 Days Past Due	18	10	7
90 and Greater Days Past Due	29	16	17
Nonperforming Loans	30	34	37
Total Past Due and Nonperforming Loans	82	66	70
Purchased Credit Impaired			1
Total Loans	8,162	8,047	7,877
Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,114	1,084	987
30 - 59 Days Past Due	10	2	3
60 - 89 Days Past Due	4		5
90 and Greater Days Past Due	2	1	1
Nonperforming Loans	12	13	11
Total Past Due and Nonperforming Loans	28	16	20
Total Loans	1,142	1,100	1,007
Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	9,194	9,065	8,793
30 - 59 Days Past Due	15	8	12
60 - 89 Days Past Due	22	10	12
90 and Greater Days Past Due	31	17	18
Nonperforming Loans	42	47	48
Total Past Due and Nonperforming Loans	110	82	90
Purchased Credit Impaired			1
Total Loans	9,304	9,147	8,884
Commercial Lease Financing [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	4,017	4,172	4,338
30 - 59 Days Past Due	9	30	15
60 - 89 Days Past Due	6	21	12
90 and Greater Days Past Due	12	11	13
Nonperforming Loans	20	18	18
Total Past Due and Nonperforming Loans	47	80	58
Total Loans	4,064	4,252	4,396
Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	41,814	41,095	39,202
30 - 59 Days Past Due	60	57	94
60 - 89 Days Past Due	39	45	32
90 and Greater Days Past Due	78	60	49
Nonperforming Loans	160	124	126
Total Past Due and Nonperforming Loans	337	286	301
Purchased Credit Impaired			1
Total Loans	42,151	41,381	39,504
Real Estate - Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,129	2,111	2,043
30 - 59 Days Past Due	12	12	13
60 - 89 Days Past Due	5	7	6
90 and Greater Days Past Due	2	4	3
Nonperforming Loans	72	79	105
Total Past Due and Nonperforming Loans	91	102	127
Purchased Credit Impaired	11	12	13
Total Loans	2,231	2,225	2,183
Home Equity: Key Community Bank [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	10,012	10,098	10,010
30 - 59 Days Past Due	39	46	45
60 - 89 Days Past Due	23	22	25
90 and Greater Days Past Due	13	14	11
Nonperforming Loans	182	185	188
Total Past Due and Nonperforming Loans	257	267	269
Purchased Credit Impaired	1	1	2
Total Loans	10,270	10,366	10,281
Home Equity: Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	238	249	296
30 - 59 Days Past Due	4	5	5
60 - 89 Days Past Due	1	2	2
90 and Greater Days Past Due	1	1	1
Nonperforming Loans	9	10	11
Total Past Due and Nonperforming Loans	15	18	19
Total Loans	253	267	315
Home Equity Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	10,250	10,347	10,306
30 - 59 Days Past Due	43	51	50
60 - 89 Days Past Due	24	24	27
90 and Greater Days Past Due	14	15	12
Nonperforming Loans	191	195	199
Total Past Due and Nonperforming Loans	272	285	288
Purchased Credit Impaired	1	1	2
Total Loans	10,523	10,633	10,596
Consumer Other - Key Community Bank [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,527	1,541	1,415
30 - 59 Days Past Due	8	9	8
60 - 89 Days Past Due	4	3	5
90 and Greater Days Past Due	6	5	6
Nonperforming Loans	2	2	2
Total Past Due and Nonperforming Loans	20	19	21
Total Loans	1,547	1,560	1,436
Consumer Credit Card Financing Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	708	733	671
30 - 59 Days Past Due	5	6	7
60 - 89 Days Past Due	3	4	4
90 and Greater Days Past Due	9	9	15
Nonperforming Loans	2	2	1
Total Past Due and Nonperforming Loans	19	21	27
Total Loans	727	754	698
Consumer Marine [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	707	746	928
30 - 59 Days Past Due	8	11	12
60 - 89 Days Past Due	4	5	6
90 and Greater Days Past Due	2	2	4
Nonperforming Loans	9	15	15
Total Past Due and Nonperforming Loans	23	33	37
Total Loans	730	779	965
Consumer Others [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	42	46	59
30 - 59 Days Past Due	1	1	2
60 - 89 Days Past Due			1
90 and Greater Days Past Due		1
Nonperforming Loans	1	1	1
Total Past Due and Nonperforming Loans	2	3	4
Total Loans	44	49	63
Consumer Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	749	792	987
30 - 59 Days Past Due	9	12	14
60 - 89 Days Past Due	4	5	7
90 and Greater Days Past Due	2	3	4
Nonperforming Loans	10	16	16
Total Past Due and Nonperforming Loans	25	36	41
Total Loans	774	828	1,028
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	15,363	15,524	15,422
30 - 59 Days Past Due	77	90	92
60 - 89 Days Past Due	40	43	49
90 and Greater Days Past Due	33	36	40
Nonperforming Loans	277	294	323
Total Past Due and Nonperforming Loans	427	463	504
Purchased Credit Impaired	12	13	15
Total Loans	$ 15,802	$ 16,000	$ 15,941