Asset Quality - Breakdown of Nonperforming TDRs by Loans Category (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015 SecurityLoan	Mar. 31, 2014 SecurityLoan	Dec. 31, 2014 SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of loans	4,740	4,858	4,776
Pre-modification Outstanding Recorded Investment	$ 343	$ 406	$ 349
Post-modification Outstanding Recorded Investment	268	294	270
Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	2,051	2,347	2,391
Pre-modification Outstanding Recorded Investment	176	249	200
Post-modification Outstanding Recorded Investment	141	178	157
Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	2,689	2,511	2,385
Pre-modification Outstanding Recorded Investment	167	157	149
Post-modification Outstanding Recorded Investment	127	116	113
Commercial, Financial and Agricultural [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	11	28	14
Pre-modification Outstanding Recorded Investment	25	58	25
Post-modification Outstanding Recorded Investment	22	33	23
Commercial, Financial and Agricultural [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	17	42	20
Pre-modification Outstanding Recorded Investment	6	7	6
Post-modification Outstanding Recorded Investment	3	4	3
Commercial Real Estate: Commercial Mortgage [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	12	11	10
Pre-modification Outstanding Recorded Investment	37	40	38
Post-modification Outstanding Recorded Investment	13	14	13
Commercial Real Estate: Commercial Mortgage [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	1	4	1
Pre-modification Outstanding Recorded Investment	2	18	2
Post-modification Outstanding Recorded Investment	1	8	1
Commercial Real Estate: Construction [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans		5	1
Pre-modification Outstanding Recorded Investment		16	5
Post-modification Outstanding Recorded Investment		2
Commercial Real Estate Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	12	16	11
Pre-modification Outstanding Recorded Investment	37	56	43
Post-modification Outstanding Recorded Investment	13	16	13
Commercial Real Estate Loans [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	1	4	1
Pre-modification Outstanding Recorded Investment	2	18	2
Post-modification Outstanding Recorded Investment	1	8	1
Commercial Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	23	44	25
Pre-modification Outstanding Recorded Investment	62	114	68
Post-modification Outstanding Recorded Investment	35	49	36
Commercial Loans [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	18	46	21
Pre-modification Outstanding Recorded Investment	8	25	8
Post-modification Outstanding Recorded Investment	4	12	4
Real Estate - Residential Mortgage [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	383	687	453
Pre-modification Outstanding Recorded Investment	22	42	27
Post-modification Outstanding Recorded Investment	22	42	27
Real Estate - Residential Mortgage [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	454	111	381
Pre-modification Outstanding Recorded Investment	34	12	29
Post-modification Outstanding Recorded Investment	34	12	29
Home Equity: Key Community Bank [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	1,071	1,190	1,184
Pre-modification Outstanding Recorded Investment	76	73	79
Post-modification Outstanding Recorded Investment	70	69	72
Home Equity: Key Community Bank [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	803	708	674
Pre-modification Outstanding Recorded Investment	47	40	41
Post-modification Outstanding Recorded Investment	41	37	36
Home Equity: Other [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	128	132	158
Pre-modification Outstanding Recorded Investment	4	4	4
Post-modification Outstanding Recorded Investment	3	4	4
Home Equity: Other [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	339	312	310
Pre-modification Outstanding Recorded Investment	10	9	9
Post-modification Outstanding Recorded Investment	8	8	8
Home Equity Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	1,199	1,322	1,342
Pre-modification Outstanding Recorded Investment	80	77	83
Post-modification Outstanding Recorded Investment	73	73	76
Home Equity Loans [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	1,142	1,020	984
Pre-modification Outstanding Recorded Investment	57	49	50
Post-modification Outstanding Recorded Investment	49	45	44
Consumer Other - Key Community Bank [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	28	33	37
Pre-modification Outstanding Recorded Investment	1	1	2
Post-modification Outstanding Recorded Investment	1	1	1
Consumer Other - Key Community Bank [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	51	51	45
Pre-modification Outstanding Recorded Investment	2	2	2
Post-modification Outstanding Recorded Investment	2	2	2
Consumer Credit Card Financing Receivable [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	275	10	290
Pre-modification Outstanding Recorded Investment	2		2
Post-modification Outstanding Recorded Investment	1		2
Consumer Credit Card Financing Receivable [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	519	785	514
Pre-modification Outstanding Recorded Investment	4	5	4
Post-modification Outstanding Recorded Investment	2	5	2
Consumer Marine [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	117	210	206
Pre-modification Outstanding Recorded Investment	8	14	17
Post-modification Outstanding Recorded Investment	8	12	14
Consumer Marine [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	429	430	373
Pre-modification Outstanding Recorded Investment	60	62	54
Post-modification Outstanding Recorded Investment	35	39	31
Other Consumer [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	26	41	38
Pre-modification Outstanding Recorded Investment	1	1	1
Post-modification Outstanding Recorded Investment	1	1	1
Other Consumer [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	76	68	67
Pre-modification Outstanding Recorded Investment	2	2	2
Post-modification Outstanding Recorded Investment	1	1	1
Consumer Other [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	143	251	244
Pre-modification Outstanding Recorded Investment	9	15	18
Post-modification Outstanding Recorded Investment	9	13	15
Consumer Other [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	505	498	440
Pre-modification Outstanding Recorded Investment	62	64	56
Post-modification Outstanding Recorded Investment	36	40	32
Consumer Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	2,028	2,303	2,366
Pre-modification Outstanding Recorded Investment	114	135	132
Post-modification Outstanding Recorded Investment	106	129	121
Consumer Loans [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of loans	2,671	2,465	2,364
Pre-modification Outstanding Recorded Investment	159	132	141
Post-modification Outstanding Recorded Investment	$ 123	$ 104	$ 109