Asset Quality - Changes in Allowance for Loan and Lease Losses by Loan Category (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period - continuing operations	$ 848	$ 888	$ 1,004
Provision	59		229
Charge-offs	(211)	(308)	(508)
Recoveries	98	140	163
Balance at end of period - continuing operations	794	848	888
Discontinued Operations, Beginning Balance	39	55	104
Provision, Discontinued Operations	21	21	9
Charge-offs, Discontinued Operations	(45)	(55)	(75)
Recoveries, Discontinued Operations	14	18	17
Discontinued Operations, Ending Balance	29	39	55
Segment, Discontinued Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Financing Receivable Allowance For Credit Losses Total ALLL Including Discontinued Operations, Beginning Balance		943	1,108
Provisions, Total ALLL - Including Discontinued Operations			238
Discontinued Operations, Beginning Balance	887	943
Provision, Discontinued Operations	80	149
Charge-offs, Discontinued Operations	(256)	(363)	(583)
Recoveries, Discontinued Operations	112	158	180
Discontinued Operations, Ending Balance	823	887	943
Financing Receivable Allowance For Credit Losses Total ALLL Including Discontinued Operations, Ending Balance			943
Commercial, Financial and Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period - continuing operations	362	327	334
Provision	41	58	10
Charge-offs	(45)	(62)	(80)
Recoveries	33	39	63
Balance at end of period - continuing operations	391	362	327
Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period - continuing operations	165	198	272
Provision	(15)	(40)	5
Charge-offs	(6)	(20)	(102)
Recoveries	4	27	23
Balance at end of period - continuing operations	148	165	198
Commercial Real Estate Construction Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period - continuing operations	32	41	63
Provision	(16)	(20)	(3)
Charge-offs	(5)	(3)	(24)
Recoveries	17	14	5
Balance at end of period - continuing operations	28	32	41
Commercial Lease Financing [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period - continuing operations	62	55	78
Provision	(6)	19	(18)
Charge-offs	(10)	(27)	(27)
Recoveries	10	15	22
Balance at end of period - continuing operations	56	62	55
Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period - continuing operations	621	621	747
Provision	4	17	(6)
Charge-offs	(66)	(112)	(233)
Recoveries	64	95	113
Balance at end of period - continuing operations	623	621	621
Real Estate - Residential Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period - continuing operations	37	30	37
Provision	(6)	25	17
Charge-offs	(10)	(20)	(27)
Recoveries	2	2	3
Balance at end of period - continuing operations	23	37	30
Home Equity: Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period - continuing operations	84	105	103
Provision	10	31	90
Charge-offs	(37)	(62)	(99)
Recoveries	9	10	11
Balance at end of period - continuing operations	66	84	105
Home Equity: Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period - continuing operations	11	25	29
Provision	(2)		26
Charge-offs	(9)	(20)	(35)
Recoveries	5	6	5
Balance at end of period - continuing operations	5	11	25
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period - continuing operations	95	130	132
Provision	8	31	116
Charge-offs	(46)	(82)	(134)
Recoveries	14	16	16
Balance at end of period - continuing operations	71	95	130
Consumer Other - Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period - continuing operations	29	38	41
Provision	17	15	29
Charge-offs	(30)	(31)	(38)
Recoveries	6	7	6
Balance at end of period - continuing operations	22	29	38
Credit Cards [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period - continuing operations	34	26
Provision	32	35	37
Charge-offs	(34)	(30)	(11)
Recoveries	1	3
Balance at end of period - continuing operations	33	34	26
Consumer Marine [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period - continuing operations	29	39	46
Provision	6	4	30
Charge-offs	(23)	(29)	(59)
Recoveries	9	15	22
Balance at end of period - continuing operations	21	29	39
Consumer Others [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period - continuing operations	3	4	1
Provision	(2)	1	6
Charge-offs	(2)	(4)	(6)
Recoveries	2	2	3
Balance at end of period - continuing operations	1	3	4
Consumer Other - Key Corporate Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period - continuing operations	32	43	47
Provision	4	5	36
Charge-offs	(25)	(33)	(65)
Recoveries	11	17	25
Balance at end of period - continuing operations	22	32	43
Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period - continuing operations	227	267	257
Provision	55	111	235
Charge-offs	(145)	(196)	(275)
Recoveries	34	45	50
Balance at end of period - continuing operations	$ 171	$ 227	$ 267