Asset Quality - Breakdown of Individually Impaired Loans (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	$ 232	$ 257	$ 258
Recorded Investment	411	428	411
With no related allowance recorded, Unpaid Principal Balance	410	555	477
Unpaid Principal Balance	600	737	639
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	275	242	238
With related allowance recorded, Recorded Investment	179	171	153
With related allowance recorded, Unpaid Principal Balance	190	182	162
Specific Allowance	47	39	35
With related allowance recorded, Average Recorded Investment	163	181	162
Average Recorded Investment	438	423	400
Commercial, Financial and Agricultural [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	58	57	32
With no related allowance recorded, Unpaid Principal Balance	116	118	64
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	74	58	60
With related allowance recorded, Recorded Investment	50	35	33
With related allowance recorded, Unpaid Principal Balance	51	45	42
Specific Allowance	17	12	12
With related allowance recorded, Average Recorded Investment	36	39	48
Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	43	106	89
With no related allowance recorded, Unpaid Principal Balance	80	182	142
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	66	109	95
With related allowance recorded, Recorded Investment	3	31	7
With related allowance recorded, Unpaid Principal Balance	3	32	7
Specific Allowance	1	7	1
With related allowance recorded, Average Recorded Investment	4	44	51
Commercial Real Estate: Construction [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	41	42	48
With no related allowance recorded, Unpaid Principal Balance	124	203	182
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	45	47	39
With related allowance recorded, Recorded Investment	3
With related allowance recorded, Unpaid Principal Balance	13
Specific Allowance
With related allowance recorded, Average Recorded Investment	2	2	6
Commercial Real Estate Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	84	148	137
With no related allowance recorded, Unpaid Principal Balance	204	385	324
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	111	156	134
With related allowance recorded, Recorded Investment	6	31	7
With related allowance recorded, Unpaid Principal Balance	16	32	7
Specific Allowance	1	7	1
With related allowance recorded, Average Recorded Investment	6	46	57
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	142	205	169
With no related allowance recorded, Unpaid Principal Balance	320	503	388
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	185	214	194
With related allowance recorded, Recorded Investment	56	66	40
With related allowance recorded, Unpaid Principal Balance	67	77	49
Specific Allowance	18	19	13
With related allowance recorded, Average Recorded Investment	42	85	105
Real Estate - Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	16		21
With no related allowance recorded, Unpaid Principal Balance	16		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	18	17
With related allowance recorded, Unpaid Principal Balance	20	18	17
Specific Allowance	6	1	1
With related allowance recorded, Average Recorded Investment	20	17	8
Home Equity: Key Community Bank [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	69	45	65
With no related allowance recorded, Unpaid Principal Balance	69	45	65
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	69	23	33
With related allowance recorded, Recorded Investment	33	20	22
With related allowance recorded, Unpaid Principal Balance	33	20	22
Specific Allowance	10	10	11
With related allowance recorded, Average Recorded Investment	32	16	11
Home Equity: Other [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	2	2	3
With no related allowance recorded, Unpaid Principal Balance	2	2	3
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	2	1	1
With related allowance recorded, Recorded Investment	11	8	9
With related allowance recorded, Unpaid Principal Balance	11	8	9
Specific Allowance	2	1	1
With related allowance recorded, Average Recorded Investment	10	7	5
Home Equity Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	71	47	68
With no related allowance recorded, Unpaid Principal Balance	71	47	68
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	71	24	34
With related allowance recorded, Recorded Investment	44	28	31
With related allowance recorded, Unpaid Principal Balance	44	28	31
Specific Allowance	12	11	12
With related allowance recorded, Average Recorded Investment	42	23	16
Consumer Other - Key Community Bank [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment		1
With no related allowance recorded, Unpaid Principal Balance		1
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment		1
With related allowance recorded, Recorded Investment	3	2	2
With related allowance recorded, Unpaid Principal Balance	3	2	2
Specific Allowance		1	2
With related allowance recorded, Average Recorded Investment	3	2	1
Consumer Credit Card [Member]
Financing Receivable, Impaired [Line Items]
With related allowance recorded, Recorded Investment	6		2
With related allowance recorded, Unpaid Principal Balance	6		2
Specific Allowance	1
With related allowance recorded, Average Recorded Investment	5		1
Consumer Marine [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	3	4
With no related allowance recorded, Unpaid Principal Balance	3	4
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	3	2
With related allowance recorded, Recorded Investment	49	56	60
With related allowance recorded, Unpaid Principal Balance	49	56	60
Specific Allowance	10	7	7
With related allowance recorded, Average Recorded Investment	50	53	30
Other Consumer [Member]
Financing Receivable, Impaired [Line Items]
With related allowance recorded, Recorded Investment	1	1	1
With related allowance recorded, Unpaid Principal Balance	1	1	1
Specific Allowance
With related allowance recorded, Average Recorded Investment	1	1	1
Consumer Other [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	3	4
With no related allowance recorded, Unpaid Principal Balance	3	4
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	3	2
With related allowance recorded, Recorded Investment	50	57	61
With related allowance recorded, Unpaid Principal Balance	50	57	61
Specific Allowance	10	7	7
With related allowance recorded, Average Recorded Investment	51	54	31
Consumer Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	90	52	89
With no related allowance recorded, Unpaid Principal Balance	90	52	89
With no related allowance recorded, Specific Allowance
With no related allowance recorded, Average Recorded Investment	90	28	44
With related allowance recorded, Recorded Investment	123	105	113
With related allowance recorded, Unpaid Principal Balance	123	105	113
Specific Allowance	29	20	22
With related allowance recorded, Average Recorded Investment	$ 121	$ 96	$ 57