Asset Quality - Past Due Loans Including Current Loans (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 52,660	$ 51,623	$ 50,298
30 - 59 Days Past Due	213	286	252
60 - 89 Days Past Due	75	138	102
90 and Greater Days Past Due	90	78	89
Nonperforming Loans	541	674	653
Total Past Due and Nonperforming Loans	919	1,176	1,096
Purchased Credit Impaired	41	23	25
Total Loans	53,597	52,822	51,419
Commercial, Financial and Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	24,161	23,030	21,766
30 - 59 Days Past Due	33	56	46
60 - 89 Days Past Due	9	34	19
90 and Greater Days Past Due	12	22	15
Nonperforming Loans	102	99	132
Total Past Due and Nonperforming Loans	156	211	212
Purchased Credit Impaired		1	1
Total Loans	24,317	23,242	21,979
Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	7,429	7,556	7,344
30 - 59 Days Past Due	22	21	19
60 - 89 Days Past Due	2	11	3
90 and Greater Days Past Due	31	9	26
Nonperforming Loans	58	120	134
Total Past Due and Nonperforming Loans	113	161	182
Purchased Credit Impaired	2	3	3
Total Loans	7,544	7,720	7,529
Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,038	943	993
30 - 59 Days Past Due	3	1	5
60 - 89 Days Past Due		2	3
90 and Greater Days Past Due		1	13
Nonperforming Loans	17	56	53
Total Past Due and Nonperforming Loans	20	60	74
Purchased Credit Impaired
Total Loans	1,058	1,003	1,067
Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	8,467	8,499	8,337
30 - 59 Days Past Due	25	22	24
60 - 89 Days Past Due	2	13	6
90 and Greater Days Past Due	31	10	39
Nonperforming Loans	75	176	187
Total Past Due and Nonperforming Loans	133	221	256
Purchased Credit Impaired	2	3	3
Total Loans	8,602	8,723	8,596
Commercial Lease Financing [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	4,472	4,772	4,881
30 - 59 Days Past Due	41	88	48
60 - 89 Days Past Due	7	31	11
90 and Greater Days Past Due	8	8	2
Nonperforming Loans	22	16	18
Total Past Due and Nonperforming Loans	78	143	79
Purchased Credit Impaired
Total Loans	4,550	4,915	4,960
Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	37,100	36,301	34,984
30 - 59 Days Past Due	99	166	118
60 - 89 Days Past Due	18	78	36
90 and Greater Days Past Due	51	40	56
Nonperforming Loans	199	291	337
Total Past Due and Nonperforming Loans	367	575	547
Purchased Credit Impaired	2	4	4
Total Loans	37,469	36,880	35,535
Real Estate - Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,045	2,023	1,997
30 - 59 Days Past Due	22	16	22
60 - 89 Days Past Due	9	10	13
90 and Greater Days Past Due	10	6	6
Nonperforming Loans	98	103	83
Total Past Due and Nonperforming Loans	139	135	124
Purchased Credit Impaired	14	16	17
Total Loans	2,198	2,174	2,138
Home Equity: Key Community Bank [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	9,994	9,506	9,492
30 - 59 Days Past Due	50	54	57
60 - 89 Days Past Due	29	26	30
90 and Greater Days Past Due	12	17	15
Nonperforming Loans	198	210	171
Total Past Due and Nonperforming Loans	289	307	273
Purchased Credit Impaired	2	3	3
Total Loans	10,285	9,816	9,768
Home Equity: Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	327	387	374
30 - 59 Days Past Due	8	9	9
60 - 89 Days Past Due	3	4	5
90 and Greater Days Past Due	2	2	3
Nonperforming Loans	13	21	18
Total Past Due and Nonperforming Loans	26	36	35
Purchased Credit Impaired
Total Loans	353	423	409
Home Equity Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	10,321	9,893	9,866
30 - 59 Days Past Due	58	63	66
60 - 89 Days Past Due	32	30	35
90 and Greater Days Past Due	14	19	18
Nonperforming Loans	211	231	189
Total Past Due and Nonperforming Loans	315	343	308
Purchased Credit Impaired	2	3	3
Total Loans	10,638	10,239	10,177
Consumer Other - Key Community Bank [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,419	1,325	1,290
30 - 59 Days Past Due	8	9	9
60 - 89 Days Past Due	5	5	4
90 and Greater Days Past Due	6	8	6
Nonperforming Loans	2	2	3
Total Past Due and Nonperforming Loans	21	24	22
Purchased Credit Impaired			1
Total Loans	1,440	1,349	1,313
Consumer Marine [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,034	1,288	1,377
30 - 59 Days Past Due	17	23	29
60 - 89 Days Past Due	6	9	9
90 and Greater Days Past Due	1	4	2
Nonperforming Loans	25	34	31
Total Past Due and Nonperforming Loans	49	70	71
Purchased Credit Impaired
Total Loans	1,083	1,358	1,448
Consumer Others [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	66	87	92
30 - 59 Days Past Due	2	2	2
60 - 89 Days Past Due	1	1	1
90 and Greater Days Past Due		1	1
Nonperforming Loans	2	2	2
Total Past Due and Nonperforming Loans	5	6	6
Purchased Credit Impaired
Total Loans	71	93	98
Consumer Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,100	1,375	1,469
30 - 59 Days Past Due	19	25	31
60 - 89 Days Past Due	7	10	10
90 and Greater Days Past Due	1	5	3
Nonperforming Loans	27	36	33
Total Past Due and Nonperforming Loans	54	76	77
Purchased Credit Impaired
Total Loans	1,154	1,451	1,546
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	15,560	15,322	15,314
30 - 59 Days Past Due	114	120	134
60 - 89 Days Past Due	57	60	66
90 and Greater Days Past Due	39	38	33
Nonperforming Loans	342	383	316
Total Past Due and Nonperforming Loans	552	601	549
Purchased Credit Impaired	16	19	21
Total Loans	16,128	15,942	15,884
Credit Cards [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	675	706	692
30 - 59 Days Past Due	7	7	6
60 - 89 Days Past Due	4	5	4
90 and Greater Days Past Due	8
Nonperforming Loans	4	11	8
Total Past Due and Nonperforming Loans	23	23	18
Purchased Credit Impaired
Total Loans	$ 698	$ 729	$ 710