Asset Quality - Changes in Allowance for Loan and Lease Losses by Loan Category (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			$ 823	$ 887
Provision			110	52
Charge-offs			(177)	(196)
Recoveries			57	92
Ending balance	$ 813	$ 835	813	835
Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	796	814	794	848
Provision			101	37
Charge-offs	(53)	(49)	(152)	(162)
Recoveries	12	18	47	81
Ending balance	790	804	790	804
Continuing Operations [Member] | Commercial, Financial and Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			391	362
Provision			93	32
Charge-offs			(59)	(35)
Recoveries			13	27
Ending balance	438	386	438	386
Continuing Operations [Member] | Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			148	165
Provision			(9)	(7)
Charge-offs			(2)	(3)
Recoveries			2	4
Ending balance	139	159	139	159
Continuing Operations [Member] | Commercial Real Estate Construction Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			28	32
Provision			(3)	(16)
Charge-offs			(1)	(4)
Recoveries			1	16
Ending balance	25	28	25	28
Continuing Operations [Member] | Commercial Lease Financing [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			56	62
Provision			(13)	(9)
Charge-offs			(5)	(6)
Recoveries			7	8
Ending balance	45	55	45	55
Continuing Operations [Member] | Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			623	621
Provision			68
Charge-offs			(67)	(48)
Recoveries			23	55
Ending balance	647	628	647	628
Continuing Operations [Member] | Real Estate - Residential Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			23	37
Provision			(1)	(10)
Charge-offs			(4)	(7)
Recoveries			1	2
Ending balance	19	22	19	22
Continuing Operations [Member] | Home Equity: Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			66	84
Provision			4	9
Charge-offs			(21)	(29)
Recoveries			5	7
Ending balance	54	71	54	71
Continuing Operations [Member] | Home Equity: Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			5	11
Provision			(1)	(1)
Charge-offs			(4)	(8)
Recoveries			4	4
Ending balance	4	6	4	6
Continuing Operations [Member] | Home Equity Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			71	95
Provision			3	8
Charge-offs			(25)	(37)
Recoveries			9	11
Ending balance	58	77	58	77
Continuing Operations [Member] | Consumer Other - Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			22	29
Provision			11	14
Charge-offs			(18)	(23)
Recoveries			5	4
Ending balance	20	24	20	24
Continuing Operations [Member] | Credit Cards [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			33	34
Provision			20	24
Charge-offs			(23)	(27)
Recoveries			2	1
Ending balance	32	32	32	32
Continuing Operations [Member] | Consumer Marine [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			21	29
Provision				1
Charge-offs			(14)	(18)
Recoveries			6	7
Ending balance	13	19	13	19
Continuing Operations [Member] | Consumer Others [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			1	3
Charge-offs			(1)	(2)
Recoveries			1	1
Ending balance	1	2	1	2
Continuing Operations [Member] | Consumer Other - Key Corporate Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			22	32
Provision				1
Charge-offs			(15)	(20)
Recoveries			7	8
Ending balance	14	21	14	21
Continuing Operations [Member] | Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			171	227
Provision			33	37
Charge-offs			(85)	(114)
Recoveries			24	26
Ending balance	143	176	143	176
Discontinued Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance			29	39
Provision			9	15
Charge-offs				(34)
Charge-offs			(25)
Recoveries			10	11
Ending balance	$ 23	$ 31	$ 23	$ 31