Asset Quality (Details 10) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Mar. 31, 2011
Financing Receivable, Allowance for Credit Loss, Additional Information
Individually Evaluated for Impairment Allowance	$ 38	$ 39
Collectively Evaluated for Impairment Allowance	906	1,333
Loans Outstanding	49,226	48,552
Individually Evaluated for Impairment corresponding allowance	359	493
Collectively Evaluated for Impairment, Outstanding	48,867	48,059
Collectively Evaluated for Impairment Allowance, Discontinued Operations	90	111
Loans Outstanding, Discontinued Operations	5,715	6,304
Collectively Evaluated for Impairment Outstanding, Discontinued Operations	5,715	6,304
Individually Evaluated for Impairment Allowance, Total ALLL - Including Discontinued Operations	38	39
Collectively Evaluated for Impairment Allowance, Total ALLL - Including Discontinued Operations	996	1,444
Loans Outstanding Total ALLL - Including Discontinued Operations	54,941	54,856
Individually Evaluated for Impairment Outstanding Total ALLL - Including Discontinued Operations		493
Collectively Evaluated for Impairment Outstanding Total ALLL - Including Discontinued Operations	54,582	54,263
Commercial, Financial and Agricultural [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information
Individually Evaluated for Impairment Allowance	19	25
Collectively Evaluated for Impairment Allowance	297	394
Loans Outstanding	19,787	16,440
Individually Evaluated for Impairment corresponding allowance	125	143
Collectively Evaluated for Impairment, Outstanding	19,662	16,297
Commercial real estate: Commercial Mortgage [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information
Individually Evaluated for Impairment Allowance	16	13
Collectively Evaluated for Impairment Allowance	247	373
Loans Outstanding	7,807	8,806
Individually Evaluated for Impairment corresponding allowance	182	218
Collectively Evaluated for Impairment, Outstanding	7,625	8,588
Commercial real estate: Construction [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information
Individually Evaluated for Impairment Allowance	3	1
Collectively Evaluated for Impairment Allowance	53	116
Loans Outstanding	1,273	1,845
Individually Evaluated for Impairment corresponding allowance	52	128
Collectively Evaluated for Impairment, Outstanding	1,221	1,717
Commercial Real Estate Loans [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information
Individually Evaluated for Impairment Allowance	19	14
Collectively Evaluated for Impairment Allowance	300	489
Loans Outstanding	9,080	10,651
Individually Evaluated for Impairment corresponding allowance	234	346
Collectively Evaluated for Impairment, Outstanding	8,846	10,305
Commercial Lease Financing [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information
Collectively Evaluated for Impairment Allowance	68	132
Loans Outstanding	5,755	6,207
Individually Evaluated for Impairment corresponding allowance		2
Collectively Evaluated for Impairment, Outstanding	5,755	6,205
Commercial [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information
Individually Evaluated for Impairment Allowance	38	39
Collectively Evaluated for Impairment Allowance	665	1,015
Loans Outstanding	34,622	33,298
Individually Evaluated for Impairment corresponding allowance	359	491
Collectively Evaluated for Impairment, Outstanding	34,263	32,807
Real estate - residential mortgage [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information
Collectively Evaluated for Impairment Allowance	36	40
Loans Outstanding	1,967	1,803
Collectively Evaluated for Impairment, Outstanding	1,967	1,803
Home Equity Key Community Bank [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information
Collectively Evaluated for Impairment Allowance	94	111
Loans Outstanding	9,153	9,421
Individually Evaluated for Impairment corresponding allowance		2
Collectively Evaluated for Impairment, Outstanding	9,153	9,419
Home Equity Other [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information
Collectively Evaluated for Impairment Allowance	28	45
Loans Outstanding	507	627
Collectively Evaluated for Impairment, Outstanding	507	627
Home Equity [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information
Collectively Evaluated for Impairment Allowance	122	156
Loans Outstanding	9,660	10,048
Individually Evaluated for Impairment corresponding allowance		2
Collectively Evaluated for Impairment, Outstanding	9,660	10,046
Consumer Other - Key Community Bank [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information
Collectively Evaluated for Impairment Allowance	37	50
Loans Outstanding	1,212	1,141
Collectively Evaluated for Impairment, Outstanding	1,212	1,141
Consumer Marine [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information
Collectively Evaluated for Impairment Allowance	45	68
Loans Outstanding	1,654	2,112
Collectively Evaluated for Impairment, Outstanding	1,654	2,112
Consumer Others [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information
Collectively Evaluated for Impairment Allowance	1	4
Loans Outstanding	111	150
Collectively Evaluated for Impairment, Outstanding	111	150
Consumer Other - Key Corporate Bank [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information
Collectively Evaluated for Impairment Allowance	46	72
Loans Outstanding	1,765	2,262
Collectively Evaluated for Impairment, Outstanding	1,765	2,262
Consumer Loans [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information
Collectively Evaluated for Impairment Allowance	241	318
Loans Outstanding	14,604	15,254
Individually Evaluated for Impairment corresponding allowance		2
Collectively Evaluated for Impairment, Outstanding	$ 14,604	$ 15,252