Asset Quality - Breakdown of Nonperforming TDRs by Loans Category (Detail) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($) SecurityLoan	Sep. 30, 2014 USD ($) SecurityLoan	Dec. 31, 2014 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	4,659	4,730	4,776
Pre-modification Outstanding Recorded Investment	$ 366	$ 364	$ 349
Post-modification Outstanding Recorded Investment	$ 287	$ 264	$ 270
Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,042	2,195	2,391
Pre-modification Outstanding Recorded Investment	$ 198	$ 191	$ 200
Post-modification Outstanding Recorded Investment	$ 159	$ 137	$ 157
Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,617	2,535	2,385
Pre-modification Outstanding Recorded Investment	$ 168	$ 173	$ 149
Post-modification Outstanding Recorded Investment	$ 128	$ 127	$ 113
Commercial, Financial and Agricultural [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	12	20	14
Pre-modification Outstanding Recorded Investment	$ 56	$ 16	$ 25
Post-modification Outstanding Recorded Investment	$ 50	$ 9	$ 23
Commercial, Financial and Agricultural [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	12	25	20
Pre-modification Outstanding Recorded Investment	$ 6	$ 6	$ 6
Post-modification Outstanding Recorded Investment	$ 3	$ 3	$ 3
Commercial Real Estate: Commercial Mortgage [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	11	12	10
Pre-modification Outstanding Recorded Investment	$ 30	$ 39	$ 38
Post-modification Outstanding Recorded Investment	$ 7	$ 14	$ 13
Commercial Real Estate: Commercial Mortgage [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	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 | SecurityLoan		3	1
Pre-modification Outstanding Recorded Investment		$ 15	$ 5
Post-modification Outstanding Recorded Investment		$ 1
Commercial Real Estate Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	11	15	11
Pre-modification Outstanding Recorded Investment	$ 30	$ 54	$ 43
Post-modification Outstanding Recorded Investment	$ 7	$ 15	$ 13
Commercial Real Estate Loans [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	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 | SecurityLoan	23	35	25
Pre-modification Outstanding Recorded Investment	$ 86	$ 70	$ 68
Post-modification Outstanding Recorded Investment	$ 57	$ 24	$ 36
Commercial Loans [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	13	29	21
Pre-modification Outstanding Recorded Investment	$ 8	$ 24	$ 8
Post-modification Outstanding Recorded Investment	$ 4	$ 11	$ 4
Real Estate - Residential Mortgage [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	356	464	453
Pre-modification Outstanding Recorded Investment	$ 21	$ 28	$ 27
Post-modification Outstanding Recorded Investment	$ 21	$ 28	$ 27
Real Estate - Residential Mortgage [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	499	359	381
Pre-modification Outstanding Recorded Investment	$ 36	$ 28	$ 29
Post-modification Outstanding Recorded Investment	$ 36	$ 28	$ 29
Home Equity: Key Community Bank [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,093	1,125	1,184
Pre-modification Outstanding Recorded Investment	$ 79	$ 70	$ 79
Post-modification Outstanding Recorded Investment	$ 70	$ 64	$ 72
Home Equity: Key Community Bank [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	794	731	674
Pre-modification Outstanding Recorded Investment	$ 49	$ 45	$ 41
Post-modification Outstanding Recorded Investment	$ 42	$ 40	$ 36
Home Equity: Other [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	122	133	158
Pre-modification Outstanding Recorded Investment	$ 3	$ 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 | SecurityLoan	327	325	310
Pre-modification Outstanding Recorded Investment	$ 10	$ 10	$ 9
Post-modification Outstanding Recorded Investment	$ 8	$ 8	$ 8
Home Equity Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,215	1,258	1,342
Pre-modification Outstanding Recorded Investment	$ 82	$ 74	$ 83
Post-modification Outstanding Recorded Investment	$ 73	$ 68	$ 76
Home Equity Loans [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,121	1,056	984
Pre-modification Outstanding Recorded Investment	$ 59	$ 55	$ 50
Post-modification Outstanding Recorded Investment	$ 50	$ 48	$ 44
Consumer Other - Key Community Bank [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	26	31	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 | SecurityLoan	45	53	45
Pre-modification Outstanding Recorded Investment	$ 2	$ 2	$ 2
Post-modification Outstanding Recorded Investment	$ 1	$ 2	$ 2
Consumer Credit Card Financing Receivable [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	314	156	290
Pre-modification Outstanding Recorded Investment	$ 2	$ 1	$ 2
Post-modification Outstanding Recorded Investment	$ 2	$ 1	$ 2
Consumer Credit Card Financing Receivable [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	473	564	514
Pre-modification Outstanding Recorded Investment	$ 2	$ 4	$ 4
Post-modification Outstanding Recorded Investment	$ 2	$ 3	$ 2
Consumer Marine [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	92	211	206
Pre-modification Outstanding Recorded Investment	$ 6	$ 16	$ 17
Post-modification Outstanding Recorded Investment	$ 5	$ 14	$ 14
Consumer Marine [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	398	402	373
Pre-modification Outstanding Recorded Investment	$ 59	$ 58	$ 54
Post-modification Outstanding Recorded Investment	$ 33	$ 34	$ 31
Other Consumer [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	16	40	38
Pre-modification Outstanding Recorded Investment		$ 1	$ 1
Post-modification Outstanding Recorded Investment		$ 1	$ 1
Other Consumer [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	68	72	67
Pre-modification Outstanding Recorded Investment	$ 2	$ 2	$ 2
Post-modification Outstanding Recorded Investment	$ 2	$ 1	$ 1
Consumer Other [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	108	251	244
Pre-modification Outstanding Recorded Investment	$ 6	$ 17	$ 18
Post-modification Outstanding Recorded Investment	$ 5	$ 15	$ 15
Consumer Other [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	466	474	440
Pre-modification Outstanding Recorded Investment	$ 61	$ 60	$ 56
Post-modification Outstanding Recorded Investment	$ 35	$ 35	$ 32
Consumer Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,019	2,160	2,366
Pre-modification Outstanding Recorded Investment	$ 112	$ 121	$ 132
Post-modification Outstanding Recorded Investment	$ 102	$ 113	$ 121
Consumer Loans [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,604	2,506	2,364
Pre-modification Outstanding Recorded Investment	$ 160	$ 149	$ 141
Post-modification Outstanding Recorded Investment	$ 124	$ 116	$ 109