Asset Quality - Breakdown of Individually Impaired Loans (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	$ 128	$ 163	$ 115
Recorded Investment	338	304	302
With no related allowance recorded, Unpaid Principal Balance	165	205	133
Unpaid Principal Balance	376	349	320
With no related allowance recorded, Specific Allowance	0	0	0
With no related allowance recorded, Average Recorded Investment	121	182	131
With related allowance recorded, Recorded Investment	210	141	187
With related allowance recorded, Unpaid Principal Balance	211	144	187
Specific Allowance	49	34	40
With related allowance recorded, Average Recorded Investment	199	149	169
Average Recorded Investment	320	331	300
Commercial, Financial and Agricultural [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	20	33	6
With no related allowance recorded, Unpaid Principal Balance	51	60	17
With no related allowance recorded, Specific Allowance	0	0	0
With no related allowance recorded, Average Recorded Investment	13	33	8
With related allowance recorded, Recorded Investment	62	7	37
With related allowance recorded, Unpaid Principal Balance	62	10	37
Specific Allowance	20	2	9
With related allowance recorded, Average Recorded Investment	50	12	28
Commercial Real Estate: Commercial Mortgage [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	14	23	15
With no related allowance recorded, Unpaid Principal Balance	19	28	20
With no related allowance recorded, Specific Allowance	0	0	0
With no related allowance recorded, Average Recorded Investment	14	22	19
With related allowance recorded, Recorded Investment	6	2	6
With related allowance recorded, Unpaid Principal Balance	7	2	6
Specific Allowance	2	1	2
With related allowance recorded, Average Recorded Investment	6	4	6
Commercial Real Estate: Construction [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	7	7	5
With no related allowance recorded, Unpaid Principal Balance	7	17	6
With no related allowance recorded, Specific Allowance	0	0	0
With no related allowance recorded, Average Recorded Investment	6	27	7
With related allowance recorded, Recorded Investment			3
With related allowance recorded, Unpaid Principal Balance			3
Specific Allowance			1
With related allowance recorded, Average Recorded Investment	1	1	2
Commercial Real Estate Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	21	30	20
With no related allowance recorded, Unpaid Principal Balance	26	45	26
With no related allowance recorded, Specific Allowance	0	0	0
With no related allowance recorded, Average Recorded Investment	20	49	26
With related allowance recorded, Recorded Investment	6	2	9
With related allowance recorded, Unpaid Principal Balance	7	2	9
Specific Allowance	2	1	3
With related allowance recorded, Average Recorded Investment	7	5	8
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	41	63	26
With no related allowance recorded, Unpaid Principal Balance	77	105	43
With no related allowance recorded, Specific Allowance	0	0	0
With no related allowance recorded, Average Recorded Investment	33	82	34
With related allowance recorded, Recorded Investment	68	9	46
With related allowance recorded, Unpaid Principal Balance	69	12	46
Specific Allowance	22	3	12
With related allowance recorded, Average Recorded Investment	57	17	36
Real Estate - Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	23	26	24
With no related allowance recorded, Unpaid Principal Balance	23	26	24
With no related allowance recorded, Specific Allowance	0	0	0
With no related allowance recorded, Average Recorded Investment	23	26	30
With related allowance recorded, Recorded Investment	32	28	31
With related allowance recorded, Unpaid Principal Balance	32	28	31
Specific Allowance	5	5	5
With related allowance recorded, Average Recorded Investment	32	28	25
Home Equity: Key Community Bank [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	62	70	62
With no related allowance recorded, Unpaid Principal Balance	62	70	63
With no related allowance recorded, Specific Allowance	0	0	0
With no related allowance recorded, Average Recorded Investment	62	69	63
With related allowance recorded, Recorded Investment	49	36	46
With related allowance recorded, Unpaid Principal Balance	49	36	46
Specific Allowance	16	16	16
With related allowance recorded, Average Recorded Investment	48	36	43
Home Equity: Other [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	1	2	1
With no related allowance recorded, Unpaid Principal Balance	2	2	1
With no related allowance recorded, Specific Allowance	0	0	0
With no related allowance recorded, Average Recorded Investment	1	2	2
With related allowance recorded, Recorded Investment	11	11	11
With related allowance recorded, Unpaid Principal Balance	11	11	11
Specific Allowance	2	2	2
With related allowance recorded, Average Recorded Investment	11	11	11
Home Equity Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	63	72	63
With no related allowance recorded, Unpaid Principal Balance	64	72	64
With no related allowance recorded, Specific Allowance	0	0	0
With no related allowance recorded, Average Recorded Investment	63	71	65
With related allowance recorded, Recorded Investment	60	47	57
With related allowance recorded, Unpaid Principal Balance	60	47	57
Specific Allowance	18	18	18
With related allowance recorded, Average Recorded Investment	59	47	54
Consumer Marine [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	1	2	2
With no related allowance recorded, Unpaid Principal Balance	1	2	2
With no related allowance recorded, Specific Allowance	0	0	0
With no related allowance recorded, Average Recorded Investment	2	3	2
With related allowance recorded, Recorded Investment	41	49	43
With related allowance recorded, Unpaid Principal Balance	41	49	43
Specific Allowance	4	7	5
With related allowance recorded, Average Recorded Investment	42	49	45
Consumer Other [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	1	2	2
With no related allowance recorded, Unpaid Principal Balance	1	2	2
With no related allowance recorded, Specific Allowance	0	0	0
With no related allowance recorded, Average Recorded Investment	2	3	2
With related allowance recorded, Recorded Investment	43	50	45
With related allowance recorded, Unpaid Principal Balance	43	50	45
Specific Allowance	4	7	5
With related allowance recorded, Average Recorded Investment	44	50	47
Consumer Loans [Member]
Financing Receivable, Impaired [Line Items]
With no related allowance recorded, Recorded Investment	87	100	89
With no related allowance recorded, Unpaid Principal Balance	88	100	90
With no related allowance recorded, Specific Allowance	0	0	0
With no related allowance recorded, Average Recorded Investment	88	100	97
With related allowance recorded, Recorded Investment	142	132	141
With related allowance recorded, Unpaid Principal Balance	142	132	141
Specific Allowance	27	31	28
With related allowance recorded, Average Recorded Investment	142	132	133
Consumer Other - Key Community Bank [Member]
Financing Receivable, Impaired [Line Items]
With related allowance recorded, Recorded Investment	3	3	4
With related allowance recorded, Unpaid Principal Balance	3	3	4
With related allowance recorded, Average Recorded Investment	3	3	3
Consumer Credit Card [Member]
Financing Receivable, Impaired [Line Items]
With related allowance recorded, Recorded Investment	4	4	4
With related allowance recorded, Unpaid Principal Balance	4	4	4
Specific Allowance		1
With related allowance recorded, Average Recorded Investment	4	4	4
Other Consumer [Member]
Financing Receivable, Impaired [Line Items]
With related allowance recorded, Recorded Investment	2	1	2
With related allowance recorded, Unpaid Principal Balance	2	1	2
With related allowance recorded, Average Recorded Investment	$ 2	$ 1	$ 2

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-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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- Definition

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 14A

-Subparagraph (b)

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

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- Definition

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- Definition

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