Asset Quality (Details 2) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012 Loan
Financing Receivable Modifications [Line Items]
Number of loans	983
Pre-modification Outstanding Recorded Investment	$ 428
Post-modification Outstanding Recorded Investment	293
Nonperforming [Member]
Financing Receivable Modifications [Line Items]
Number of loans	284
Pre-modification Outstanding Recorded Investment	279
Post-modification Outstanding Recorded Investment	184
Prior Year Accruing [Member]
Financing Receivable Modifications [Line Items]
Number of loans	699
Pre-modification Outstanding Recorded Investment	149
Post-modification Outstanding Recorded Investment	109
Commercial, financial and agricultural [Member] | Nonperforming [Member]
Financing Receivable Modifications [Line Items]
Number of loans	102
Pre-modification Outstanding Recorded Investment	105
Post-modification Outstanding Recorded Investment	64
Commercial, financial and agricultural [Member] | Prior Year Accruing [Member]
Financing Receivable Modifications [Line Items]
Number of loans	176
Pre-modification Outstanding Recorded Investment	20
Post-modification Outstanding Recorded Investment	11
Commercial real estate: Commercial Mortgage [Member] | Nonperforming [Member]
Financing Receivable Modifications [Line Items]
Number of loans	16
Pre-modification Outstanding Recorded Investment	102
Post-modification Outstanding Recorded Investment	64
Commercial real estate: Commercial Mortgage [Member] | Prior Year Accruing [Member]
Financing Receivable Modifications [Line Items]
Number of loans	7
Pre-modification Outstanding Recorded Investment	75
Post-modification Outstanding Recorded Investment	57
Commercial real estate: Construction [Member] | Nonperforming [Member]
Financing Receivable Modifications [Line Items]
Number of loans	8
Pre-modification Outstanding Recorded Investment	35
Post-modification Outstanding Recorded Investment	19
Commercial real estate: Construction [Member] | Prior Year Accruing [Member]
Financing Receivable Modifications [Line Items]
Number of loans	1
Pre-modification Outstanding Recorded Investment	15
Post-modification Outstanding Recorded Investment	2
Commercial Real Estate Loans [Member] | Nonperforming [Member]
Financing Receivable Modifications [Line Items]
Number of loans	24
Pre-modification Outstanding Recorded Investment	137
Post-modification Outstanding Recorded Investment	83
Commercial Real Estate Loans [Member] | Prior Year Accruing [Member]
Financing Receivable Modifications [Line Items]
Number of loans	8
Pre-modification Outstanding Recorded Investment	90
Post-modification Outstanding Recorded Investment	59
Commercial Loans [Member] | Nonperforming [Member]
Financing Receivable Modifications [Line Items]
Number of loans	126
Pre-modification Outstanding Recorded Investment	242
Post-modification Outstanding Recorded Investment	147
Commercial Loans [Member] | Prior Year Accruing [Member]
Financing Receivable Modifications [Line Items]
Number of loans	184
Pre-modification Outstanding Recorded Investment	110
Post-modification Outstanding Recorded Investment	70
Real estate - residential mortgage [Member] | Nonperforming [Member]
Financing Receivable Modifications [Line Items]
Number of loans	43
Pre-modification Outstanding Recorded Investment	5
Post-modification Outstanding Recorded Investment	5
Real estate - residential mortgage [Member] | Prior Year Accruing [Member]
Financing Receivable Modifications [Line Items]
Number of loans	113
Pre-modification Outstanding Recorded Investment	12
Post-modification Outstanding Recorded Investment	12
Home Equity Key Community Bank [Member] | Nonperforming [Member]
Financing Receivable Modifications [Line Items]
Number of loans	27
Pre-modification Outstanding Recorded Investment	3
Post-modification Outstanding Recorded Investment	3
Home Equity Key Community Bank [Member] | Prior Year Accruing [Member]
Financing Receivable Modifications [Line Items]
Number of loans	88
Pre-modification Outstanding Recorded Investment	7
Post-modification Outstanding Recorded Investment	7
Home Equity Other [Member] | Nonperforming [Member]
Financing Receivable Modifications [Line Items]
Number of loans	32
Pre-modification Outstanding Recorded Investment	1
Post-modification Outstanding Recorded Investment	1
Home Equity Other [Member] | Prior Year Accruing [Member]
Financing Receivable Modifications [Line Items]
Number of loans	104
Pre-modification Outstanding Recorded Investment	3
Post-modification Outstanding Recorded Investment	3
Home Equity Loans [Member] | Nonperforming [Member]
Financing Receivable Modifications [Line Items]
Number of loans	59
Pre-modification Outstanding Recorded Investment	4
Post-modification Outstanding Recorded Investment	4
Home Equity Loans [Member] | Prior Year Accruing [Member]
Financing Receivable Modifications [Line Items]
Number of loans	192
Pre-modification Outstanding Recorded Investment	10
Post-modification Outstanding Recorded Investment	10
Consumer Other - Key Community Bank [Member] | Nonperforming [Member]
Financing Receivable Modifications [Line Items]
Number of loans	2
Consumer Other - Key Community Bank [Member] | Prior Year Accruing [Member]
Financing Receivable Modifications [Line Items]
Number of loans	19
Consumer Marine [Member] | Nonperforming [Member]
Financing Receivable Modifications [Line Items]
Number of loans	48
Pre-modification Outstanding Recorded Investment	28
Post-modification Outstanding Recorded Investment	28
Consumer Marine [Member] | Prior Year Accruing [Member]
Financing Receivable Modifications [Line Items]
Number of loans	140
Pre-modification Outstanding Recorded Investment	15
Post-modification Outstanding Recorded Investment	15
Other Consumer [Member] | Nonperforming [Member]
Financing Receivable Modifications [Line Items]
Number of loans	6
Other Consumer [Member] | Prior Year Accruing [Member]
Financing Receivable Modifications [Line Items]
Number of loans	51
Pre-modification Outstanding Recorded Investment	2
Post-modification Outstanding Recorded Investment	2
Consumer Other [Member] | Nonperforming [Member]
Financing Receivable Modifications [Line Items]
Number of loans	54
Pre-modification Outstanding Recorded Investment	28
Post-modification Outstanding Recorded Investment	28
Consumer Other [Member] | Prior Year Accruing [Member]
Financing Receivable Modifications [Line Items]
Number of loans	191
Pre-modification Outstanding Recorded Investment	17
Post-modification Outstanding Recorded Investment	17
Consumer Loans [Member] | Nonperforming [Member]
Financing Receivable Modifications [Line Items]
Number of loans	158
Pre-modification Outstanding Recorded Investment	37
Post-modification Outstanding Recorded Investment	37
Consumer Loans [Member] | Prior Year Accruing [Member]
Financing Receivable Modifications [Line Items]
Number of loans	515
Pre-modification Outstanding Recorded Investment	39
Post-modification Outstanding Recorded Investment	$ 39