Asset Quality - Breakdown of Individually Impaired Loans (Detail) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	$ 127	$ 146	$ 115
Recorded Investment	318	298	302
With no related allowance recorded, Unpaid Principal Balance	154	171	133
Unpaid Principal Balance	359	324	320
With no related allowance recorded, Specific Allowance	0
With no related allowance recorded, Average Recorded Investment	119	141	131
With related allowance recorded, Recorded Investment	191	152	187
With related allowance recorded, Unpaid Principal Balance	205	153	187
Specific Allowance	36	38	40
With related allowance recorded, Average Recorded Investment	207	146	169
Average Recorded Investment	326	287	300
Commercial, Financial and Agricultural [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	30	11	6
With no related allowance recorded, Unpaid Principal Balance	54	20	17
With no related allowance recorded, Specific Allowance	0
With no related allowance recorded, Average Recorded Investment	19	12	8
With related allowance recorded, Recorded Investment	43	20	37
With related allowance recorded, Unpaid Principal Balance	56	21	37
Specific Allowance	9	7	9
With related allowance recorded, Average Recorded Investment	58	12	28
Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	9	22	15
With no related allowance recorded, Unpaid Principal Balance	12	27	20
With no related allowance recorded, Specific Allowance	0
With no related allowance recorded, Average Recorded Investment	10	23	19
With related allowance recorded, Recorded Investment	5	7	6
With related allowance recorded, Unpaid Principal Balance	6	7	6
Specific Allowance	1	2	2
With related allowance recorded, Average Recorded Investment	6	5	6
Commercial Real Estate: Construction [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	5	9	5
With no related allowance recorded, Unpaid Principal Balance	5	20	6
With no related allowance recorded, Specific Allowance	0
With no related allowance recorded, Average Recorded Investment	6	7	7
With related allowance recorded, Recorded Investment			3
With related allowance recorded, Unpaid Principal Balance			3
Specific Allowance			1
With related allowance recorded, Average Recorded Investment			2
Commercial Real Estate Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	14	31	20
With no related allowance recorded, Unpaid Principal Balance	17	47	26
With no related allowance recorded, Specific Allowance	0
With no related allowance recorded, Average Recorded Investment	16	30	26
With related allowance recorded, Recorded Investment	5	7	9
With related allowance recorded, Unpaid Principal Balance	6	7	9
Specific Allowance	1	2	3
With related allowance recorded, Average Recorded Investment	6	5	8
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	44	42	26
With no related allowance recorded, Unpaid Principal Balance	71	67	43
With no related allowance recorded, Specific Allowance	0
With no related allowance recorded, Average Recorded Investment	35	42	34
With related allowance recorded, Recorded Investment	48	27	46
With related allowance recorded, Unpaid Principal Balance	62	28	46
Specific Allowance	10	9	12
With related allowance recorded, Average Recorded Investment	64	17	36
Real Estate - Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	22	36	24
With no related allowance recorded, Unpaid Principal Balance	22	36	24
With no related allowance recorded, Specific Allowance	0
With no related allowance recorded, Average Recorded Investment	22	30	30
With related allowance recorded, Recorded Investment	33	19	31
With related allowance recorded, Unpaid Principal Balance	33	19	31
Specific Allowance	5	4	5
With related allowance recorded, Average Recorded Investment	33	24	25
Home Equity: Key Community Bank [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	58	64	62
With no related allowance recorded, Unpaid Principal Balance	58	64	63
With no related allowance recorded, Specific Allowance	0
With no related allowance recorded, Average Recorded Investment	59	65	63
With related allowance recorded, Recorded Investment	54	41	46
With related allowance recorded, Unpaid Principal Balance	54	41	46
Specific Allowance	17	16	16
With related allowance recorded, Average Recorded Investment	53	39	43
Home Equity: Other [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	2	2	1
With no related allowance recorded, Unpaid Principal Balance	2	2	1
With no related allowance recorded, Specific Allowance	0
With no related allowance recorded, Average Recorded Investment	2	2	2
With related allowance recorded, Recorded Investment	10	11	11
With related allowance recorded, Unpaid Principal Balance	10	11	11
Specific Allowance	1	2	2
With related allowance recorded, Average Recorded Investment	10	11	11
Home Equity Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	60	66	63
With no related allowance recorded, Unpaid Principal Balance	60	66	64
With no related allowance recorded, Specific Allowance	0
With no related allowance recorded, Average Recorded Investment	61	67	65
With related allowance recorded, Recorded Investment	64	52	57
With related allowance recorded, Unpaid Principal Balance	64	52	57
Specific Allowance	18	18	18
With related allowance recorded, Average Recorded Investment	63	50	54
Consumer Marine [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	1	2	2
With no related allowance recorded, Unpaid Principal Balance	1	2	2
With no related allowance recorded, Specific Allowance	0
With no related allowance recorded, Average Recorded Investment	1	2	2
With related allowance recorded, Recorded Investment	38	46	43
With related allowance recorded, Unpaid Principal Balance	38	46	43
Specific Allowance	2	5	5
With related allowance recorded, Average Recorded Investment	39	47	45
Consumer Other [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	1	2	2
With no related allowance recorded, Unpaid Principal Balance	1	2	2
With no related allowance recorded, Specific Allowance	0
With no related allowance recorded, Average Recorded Investment	1	2	2
With related allowance recorded, Recorded Investment	40	48	45
With related allowance recorded, Unpaid Principal Balance	40	48	45
Specific Allowance	2	6	5
With related allowance recorded, Average Recorded Investment	41	49	47
Consumer Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	83	104	89
With no related allowance recorded, Unpaid Principal Balance	83	104	90
With no related allowance recorded, Specific Allowance	0
With no related allowance recorded, Average Recorded Investment	84	99	97
With related allowance recorded, Recorded Investment	143	125	141
With related allowance recorded, Unpaid Principal Balance	143	125	141
Specific Allowance	26	29	28
With related allowance recorded, Average Recorded Investment	143	129	133
Consumer Other - Key Community Bank [Member]
Financing Receivable, Impaired [Line Items]
With related allowance recorded, Recorded Investment	3	3	4
With related allowance recorded, Unpaid Principal Balance	3	3	4
Specific Allowance	0
With related allowance recorded, Average Recorded Investment	3	3	3
Consumer Credit Card [Member]
Financing Receivable, Impaired [Line Items]
With related allowance recorded, Recorded Investment	3	3	4
With related allowance recorded, Unpaid Principal Balance	3	3	4
Specific Allowance	1	1
With related allowance recorded, Average Recorded Investment	3	3	4
Other Consumer [Member]
Financing Receivable, Impaired [Line Items]
With related allowance recorded, Recorded Investment	2	2	2
With related allowance recorded, Unpaid Principal Balance	2	2	2
Specific Allowance	0	1
With related allowance recorded, Average Recorded Investment	$ 2	$ 2	$ 2