Asset Quality - Changes in Allowance for Loan and Lease Losses by Loan Category (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 823	$ 887
Provision	30	10
Charge-offs	(57)	(70)
Recoveries	(23)	(41)
Ending balance	819	868
Continuing Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	794	848
Provision	28	6
Charge-offs	(47)	(57)
Recoveries	19	37
Ending balance	794	834
Continuing Operations [Member] | Commercial, Financial and Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	391	362
Provision	21	13
Charge-offs	(12)	(12)
Recoveries	5	10
Ending balance	405	373
Continuing Operations [Member] | Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	148	165
Provision		(3)
Charge-offs	(2)	(2)
Recoveries	2	1
Ending balance	148	161
Continuing Operations [Member] | Commercial Real Estate Construction Financing Receivable [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	28	32
Provision	1	(7)
Charge-offs	(1)	(2)
Recoveries		14
Ending balance	28	37
Continuing Operations [Member] | Commercial Lease Financing [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	56	62
Provision	(3)	1
Charge-offs	(2)	(3)
Recoveries	4	2
Ending balance	55	62
Continuing Operations [Member] | Commercial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	623	621
Provision	19	4
Charge-offs	(17)	(19)
Recoveries	11	27
Ending balance	636	633
Continuing Operations [Member] | Real Estate - Residential Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	23	37
Provision		(8)
Charge-offs	(2)	(3)
Recoveries		1
Ending balance	21	27
Continuing Operations [Member] | Home Equity: Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	66	84
Provision	(3)	3
Charge-offs	(7)	(10)
Recoveries	2	3
Ending balance	58	80
Continuing Operations [Member] | Home Equity: Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	5	11
Provision		2
Charge-offs	(1)	(3)
Recoveries	1	1
Ending balance	5	11
Continuing Operations [Member] | Home Equity Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	71	95
Provision	(3)	5
Charge-offs	(8)	(13)
Recoveries	3	4
Ending balance	63	91
Continuing Operations [Member] | Consumer Other - Key Community Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	22	29
Provision	3	2
Charge-offs	(6)	(8)
Recoveries	2	2
Ending balance	21	25
Continuing Operations [Member] | Credit Cards [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	33	34
Provision	7	4
Charge-offs	(8)	(6)
Ending balance	32	32
Continuing Operations [Member] | Consumer Marine [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	21	29
Provision	1	(2)
Charge-offs	(5)	(7)
Recoveries	3	3
Ending balance	20	23
Continuing Operations [Member] | Consumer Others [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1	3
Provision	1	1
Charge-offs	(1)	(1)
Ending balance	1	3
Continuing Operations [Member] | Consumer Other - Key Corporate Bank [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	22	32
Provision	2	(1)
Charge-offs	(6)	(8)
Recoveries	3	3
Ending balance	21	26
Continuing Operations [Member] | Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	171	227
Provision	9	2
Charge-offs	(30)	(38)
Recoveries	8	10
Ending balance	158	201
Discontinued Operations [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	29	39
Provision	2	4
Charge-offs		(13)
Charge-offs	(10)
Recoveries	(4)	(4)
Ending balance	$ 25	$ 34