Asset Quality - Breakdown of Individually Impaired Loans (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	$ 317	$ 223	$ 258
Recorded Investment	464	411	411
With no related allowance recorded, Unpaid Principal Balance	525	546	477
Unpaid Principal Balance	692	787	639
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	315	231	238
With related allowance recorded, Recorded Investment	147	188	153
With related allowance recorded, Unpaid Principal Balance	167	241	162
Specific Allowance	34	48	35
With related allowance recorded, Average Recorded Investment	140	156	162
Average Recorded Investment	455	387	400
Commercial, Financial and Agricultural [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	89	59	32
With no related allowance recorded, Unpaid Principal Balance	140	142	64
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	91	68	60
With related allowance recorded, Recorded Investment	22	43	33
With related allowance recorded, Unpaid Principal Balance	31	53	42
Specific Allowance	6	12	12
With related allowance recorded, Average Recorded Investment	18	46	48
Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	88	112	89
With no related allowance recorded, Unpaid Principal Balance	138	199	142
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	88	113	95
With related allowance recorded, Recorded Investment	5	56	7
With related allowance recorded, Unpaid Principal Balance	6	98	7
Specific Allowance	2	15	1
With related allowance recorded, Average Recorded Investment	7	63	51
Commercial Real Estate: Construction [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	50	51	48
With no related allowance recorded, Unpaid Principal Balance	157	204	182
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	49	49	39
With related allowance recorded, Recorded Investment	2	4
With related allowance recorded, Unpaid Principal Balance	12	4
Specific Allowance		3
With related allowance recorded, Average Recorded Investment	1	4	6
Commercial Real Estate Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	138	163	137
With no related allowance recorded, Unpaid Principal Balance	295	403	324
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	137	162	134
With related allowance recorded, Recorded Investment	7	60	7
With related allowance recorded, Unpaid Principal Balance	18	102	7
Specific Allowance	2	18	1
With related allowance recorded, Average Recorded Investment	8	67	57
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	227	222	169
With no related allowance recorded, Unpaid Principal Balance	435	545	388
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	228	230	194
With related allowance recorded, Recorded Investment	29	103	40
With related allowance recorded, Unpaid Principal Balance	49	155	49
Specific Allowance	8	30	13
With related allowance recorded, Average Recorded Investment	26	113	105
Real Estate - Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	16	1	21
With no related allowance recorded, Unpaid Principal Balance	16	1	21
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	16	1	10
With related allowance recorded, Recorded Investment	20	16	17
With related allowance recorded, Unpaid Principal Balance	20	17	17
Specific Allowance	5	2	1
With related allowance recorded, Average Recorded Investment	19	8	8
Home Equity: Key Community Bank [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	69		65
With no related allowance recorded, Unpaid Principal Balance	69		65
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	66		33
With related allowance recorded, Recorded Investment	30	11	22
With related allowance recorded, Unpaid Principal Balance	30	11	22
Specific Allowance	9	3	11
With related allowance recorded, Average Recorded Investment	28	6	11
Home Equity: Other [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	2		3
With no related allowance recorded, Unpaid Principal Balance	2		3
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	2		1
With related allowance recorded, Recorded Investment	10	6	9
With related allowance recorded, Unpaid Principal Balance	10	6	9
Specific Allowance	1	1	1
With related allowance recorded, Average Recorded Investment	10	3	5
Home Equity Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	71		68
With no related allowance recorded, Unpaid Principal Balance	71		68
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	68		34
With related allowance recorded, Recorded Investment	40	17	31
With related allowance recorded, Unpaid Principal Balance	40	17	31
Specific Allowance	10	4	12
With related allowance recorded, Average Recorded Investment	38	9	16
Consumer Other - Key Community Bank [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Specific Allowance
With related allowance recorded, Recorded Investment	3	2	2
With related allowance recorded, Unpaid Principal Balance	3	2	2
Specific Allowance	1	1	2
With related allowance recorded, Average Recorded Investment	3	1	1
Consumer Credit Card [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Specific Allowance
With related allowance recorded, Recorded Investment	4		2
With related allowance recorded, Unpaid Principal Balance	4		2
Specific Allowance
With related allowance recorded, Average Recorded Investment	4		1
Consumer Marine [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	3
With no related allowance recorded, Unpaid Principal Balance	3
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	3
With related allowance recorded, Recorded Investment	50	50	60
With related allowance recorded, Unpaid Principal Balance	50	50	60
Specific Allowance	10	11	7
With related allowance recorded, Average Recorded Investment	49	25	30
Other Consumer [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	3
With no related allowance recorded, Unpaid Principal Balance	3
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	3
With related allowance recorded, Recorded Investment	1		1
With related allowance recorded, Unpaid Principal Balance	1		1
Specific Allowance
With related allowance recorded, Average Recorded Investment	1		1
Consumer Other [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Specific Allowance
With related allowance recorded, Recorded Investment	51	50	61
With related allowance recorded, Unpaid Principal Balance	51	50	61
Specific Allowance	10	11	7
With related allowance recorded, Average Recorded Investment	50	25	31
Consumer Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	90	1	89
With no related allowance recorded, Unpaid Principal Balance	90	1	89
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	87	1	44
With related allowance recorded, Recorded Investment	118	85	113
With related allowance recorded, Unpaid Principal Balance	118	86	113
Specific Allowance	26	18	22
With related allowance recorded, Average Recorded Investment	114	43	57
Commercial Lease Financing [Member]
Financing Receivable, Impaired [Line Items]
With related allowance recorded, Recorded Investment
With related allowance recorded, Unpaid Principal Balance
Specific Allowance
With related allowance recorded, Average Recorded Investment