Asset Quality - Breakdown of Individually Impaired Loans (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	$ 115	$ 201
Recorded Investment	302	358
With no related allowance recorded, Unpaid Principal Balance	133	324
Unpaid Principal Balance	320	495
With no related allowance recorded, Specific Allowance	0	0
With no related allowance recorded, Average Recorded Investment	131	230
With related allowance recorded, Recorded Investment	187	157
With related allowance recorded, Unpaid Principal Balance	187	171
Specific Allowance	40	42
With related allowance recorded, Average Recorded Investment	169	155
Average Recorded Investment	300	385
Commercial, Financial and Agricultural [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	6	33
With no related allowance recorded, Unpaid Principal Balance	17	69
With no related allowance recorded, Specific Allowance	0	0
With no related allowance recorded, Average Recorded Investment	8	33
With related allowance recorded, Recorded Investment	37	17
With related allowance recorded, Unpaid Principal Balance	37	20
Specific Allowance	9	8
With related allowance recorded, Average Recorded Investment	28	25
Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	15	21
With no related allowance recorded, Unpaid Principal Balance	20	25
With no related allowance recorded, Specific Allowance	0	0
With no related allowance recorded, Average Recorded Investment	19	55
With related allowance recorded, Recorded Investment	6	6
With related allowance recorded, Unpaid Principal Balance	6	6
Specific Allowance	2	2
With related allowance recorded, Average Recorded Investment	6	7
Commercial Real Estate: Construction [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	5	48
With no related allowance recorded, Unpaid Principal Balance	6	131
With no related allowance recorded, Specific Allowance	0	0
With no related allowance recorded, Average Recorded Investment	7	48
With related allowance recorded, Recorded Investment	3	2
With related allowance recorded, Unpaid Principal Balance	3	12
Specific Allowance	1
With related allowance recorded, Average Recorded Investment	2	1
Commercial Real Estate Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	20	69
With no related allowance recorded, Unpaid Principal Balance	26	156
With no related allowance recorded, Specific Allowance	0	0
With no related allowance recorded, Average Recorded Investment	26	103
With related allowance recorded, Recorded Investment	9	8
With related allowance recorded, Unpaid Principal Balance	9	18
Specific Allowance	3	2
With related allowance recorded, Average Recorded Investment	8	8
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	26	102
With no related allowance recorded, Unpaid Principal Balance	43	225
With no related allowance recorded, Specific Allowance	0	0
With no related allowance recorded, Average Recorded Investment	34	136
With related allowance recorded, Recorded Investment	46	25
With related allowance recorded, Unpaid Principal Balance	46	38
Specific Allowance	12	10
With related allowance recorded, Average Recorded Investment	36	33
Real Estate - Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	24	27
With no related allowance recorded, Unpaid Principal Balance	24	27
With no related allowance recorded, Specific Allowance	0	0
With no related allowance recorded, Average Recorded Investment	30	24
With related allowance recorded, Recorded Investment	31	29
With related allowance recorded, Unpaid Principal Balance	31	29
Specific Allowance	5	9
With related allowance recorded, Average Recorded Investment	25	23
Home Equity: Key Community Bank [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	62	67
With no related allowance recorded, Unpaid Principal Balance	63	67
With no related allowance recorded, Specific Allowance	0	0
With no related allowance recorded, Average Recorded Investment	63	66
With related allowance recorded, Recorded Investment	46	35
With related allowance recorded, Unpaid Principal Balance	46	35
Specific Allowance	16	10
With related allowance recorded, Average Recorded Investment	43	29
Home Equity: Other [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	1	2
With no related allowance recorded, Unpaid Principal Balance	1	2
With no related allowance recorded, Specific Allowance	0	0
With no related allowance recorded, Average Recorded Investment	2	2
With related allowance recorded, Recorded Investment	11	10
With related allowance recorded, Unpaid Principal Balance	11	11
Specific Allowance	2	1
With related allowance recorded, Average Recorded Investment	11	9
Home Equity Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	63	69
With no related allowance recorded, Unpaid Principal Balance	64	69
With no related allowance recorded, Specific Allowance	0	0
With no related allowance recorded, Average Recorded Investment	65	68
With related allowance recorded, Recorded Investment	57	45
With related allowance recorded, Unpaid Principal Balance	57	46
Specific Allowance	18	11
With related allowance recorded, Average Recorded Investment	54	38
Consumer Marine [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	2	3
With no related allowance recorded, Unpaid Principal Balance	2	3
With no related allowance recorded, Specific Allowance	0	0
With no related allowance recorded, Average Recorded Investment	2	2
With related allowance recorded, Recorded Investment	43	49
With related allowance recorded, Unpaid Principal Balance	43	49
Specific Allowance	5	10
With related allowance recorded, Average Recorded Investment	45	55
Consumer Other [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	2	3
With no related allowance recorded, Unpaid Principal Balance	2	3
With no related allowance recorded, Specific Allowance	0	0
With no related allowance recorded, Average Recorded Investment	2	2
With related allowance recorded, Recorded Investment	45	50
With related allowance recorded, Unpaid Principal Balance	45	50
Specific Allowance	5	10
With related allowance recorded, Average Recorded Investment	47	56
Consumer Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	89	99
With no related allowance recorded, Unpaid Principal Balance	90	99
With no related allowance recorded, Specific Allowance	0	0
With no related allowance recorded, Average Recorded Investment	97	94
With related allowance recorded, Recorded Investment	141	132
With related allowance recorded, Unpaid Principal Balance	141	133
Specific Allowance	28	32
With related allowance recorded, Average Recorded Investment	133	122
Consumer Other - Key Community Bank [Member]
Financing Receivable, Impaired [Line Items]
With related allowance recorded, Recorded Investment	4	3
With related allowance recorded, Unpaid Principal Balance	4	3
Specific Allowance		1
With related allowance recorded, Average Recorded Investment	3	2
Consumer Credit Card [Member]
Financing Receivable, Impaired [Line Items]
With related allowance recorded, Recorded Investment	4	5
With related allowance recorded, Unpaid Principal Balance	4	5
Specific Allowance		1
With related allowance recorded, Average Recorded Investment	4	3
Other Consumer [Member]
Financing Receivable, Impaired [Line Items]
With related allowance recorded, Recorded Investment	2	1
With related allowance recorded, Unpaid Principal Balance	2	1
Specific Allowance
With related allowance recorded, Average Recorded Investment	$ 2	$ 1

Impaired financing receivable with no related allowance related allowance.

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- Definition

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 17

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5258-111524

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 14A

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-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953550-111524

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