Asset Quality - Past Due Loans Including Current Loans (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 56,619	$ 53,544
30 - 59 Days Past Due	147	213
60 - 89 Days Past Due	88	105
90 and Greater Days Past Due	96	71
Nonperforming Loans	418	508
Total Past Due and Nonperforming Loans	749	897
Purchased Credit Impaired	13	16
Total Loans	57,381	54,457
Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	27,858	24,823
30 - 59 Days Past Due	19	39
60 - 89 Days Past Due	14	8
90 and Greater Days Past Due	32	16
Nonperforming Loans	59	77
Total Past Due and Nonperforming Loans	124	140
Total Loans	27,982	24,963
Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	7,981	7,638
30 - 59 Days Past Due	6	20
60 - 89 Days Past Due	10	7
90 and Greater Days Past Due	16	17
Nonperforming Loans	34	37
Total Past Due and Nonperforming Loans	66	81
Purchased Credit Impaired		1
Total Loans	8,047	7,720
Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,084	1,068
30 - 59 Days Past Due	2	10
90 and Greater Days Past Due	1	1
Nonperforming Loans	13	14
Total Past Due and Nonperforming Loans	16	25
Total Loans	1,100	1,093
Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	9,065	8,706
30 - 59 Days Past Due	8	30
60 - 89 Days Past Due	10	7
90 and Greater Days Past Due	17	18
Nonperforming Loans	47	51
Total Past Due and Nonperforming Loans	82	106
Purchased Credit Impaired		1
Total Loans	9,147	8,813
Commercial Lease Financing [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	4,172	4,463
30 - 59 Days Past Due	30	32
60 - 89 Days Past Due	21	33
90 and Greater Days Past Due	11	4
Nonperforming Loans	18	19
Total Past Due and Nonperforming Loans	80	88
Total Loans	4,252	4,551
Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	41,095	37,992
30 - 59 Days Past Due	57	101
60 - 89 Days Past Due	45	48
90 and Greater Days Past Due	60	38
Nonperforming Loans	124	147
Total Past Due and Nonperforming Loans	286	334
Purchased Credit Impaired		1
Total Loans	41,381	38,327
Real Estate - Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,111	2,038
30 - 59 Days Past Due	12	19
60 - 89 Days Past Due	7	5
90 and Greater Days Past Due	4	4
Nonperforming Loans	79	107
Total Past Due and Nonperforming Loans	102	135
Purchased Credit Impaired	12	14
Total Loans	2,225	2,187
Home Equity: Key Community Bank [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	10,098	10,038
30 - 59 Days Past Due	46	51
60 - 89 Days Past Due	22	31
90 and Greater Days Past Due	14	14
Nonperforming Loans	185	205
Total Past Due and Nonperforming Loans	267	301
Purchased Credit Impaired	1	1
Total Loans	10,366	10,340
Home Equity: Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	249	308
30 - 59 Days Past Due	5	6
60 - 89 Days Past Due	2	4
90 and Greater Days Past Due	1	1
Nonperforming Loans	10	15
Total Past Due and Nonperforming Loans	18	26
Total Loans	267	334
Home Equity Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	10,347	10,346
30 - 59 Days Past Due	51	57
60 - 89 Days Past Due	24	35
90 and Greater Days Past Due	15	15
Nonperforming Loans	195	220
Total Past Due and Nonperforming Loans	285	327
Purchased Credit Impaired	1	1
Total Loans	10,633	10,674
Consumer Other - Key Community Bank [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,541	1,426
30 - 59 Days Past Due	9	8
60 - 89 Days Past Due	3	5
90 and Greater Days Past Due	5	7
Nonperforming Loans	2	3
Total Past Due and Nonperforming Loans	19	23
Total Loans	1,560	1,449
Consumer Credit Card Financing Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	733	698
30 - 59 Days Past Due	6	11
60 - 89 Days Past Due	4	5
90 and Greater Days Past Due	9	4
Nonperforming Loans	2	4
Total Past Due and Nonperforming Loans	21	24
Total Loans	754	722
Consumer Marine [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	746	979
30 - 59 Days Past Due	11	15
60 - 89 Days Past Due	5	6
90 and Greater Days Past Due	2	2
Nonperforming Loans	15	26
Total Past Due and Nonperforming Loans	33	49
Total Loans	779	1,028
Consumer Others [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	46	65
30 - 59 Days Past Due	1	2
60 - 89 Days Past Due		1
90 and Greater Days Past Due	1	1
Nonperforming Loans	1	1
Total Past Due and Nonperforming Loans	3	5
Total Loans	49	70
Consumer Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	792	1,044
30 - 59 Days Past Due	12	17
60 - 89 Days Past Due	5	7
90 and Greater Days Past Due	3	3
Nonperforming Loans	16	27
Total Past Due and Nonperforming Loans	36	54
Total Loans	828	1,098
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	15,524	15,552
30 - 59 Days Past Due	90	112
60 - 89 Days Past Due	43	57
90 and Greater Days Past Due	36	33
Nonperforming Loans	294	361
Total Past Due and Nonperforming Loans	463	563
Purchased Credit Impaired	13	15
Total Loans	$ 16,000	$ 16,130