Line of Business Results - Financial Information of Business Groups (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 571	$ 563
Noninterest income	437	435
Provision (credit) for loan and lease losses	6	2
Other noninterest expense	84	77
INCOME (LOSS) FROM CONTINUING OPERATIONS BEFORE INCOME TAXES	304	330
Allocated income taxes and TE adjustments	74	92
Income (loss) from continuing operations	230	238
Income (loss) from discontinued operations, net of taxes	5	4
Net income (loss)	235	242
Less: Net income (loss) attributable to noncontrolling interests	2
Net income (loss) attributable to Key	233	242
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	575	569
Noninterest income	442	434
Total revenue (TE)	1,017	1,003
Provision (credit) for loan and lease losses	34	4
Depreciation and amortization expense	27	27
Other noninterest expense	645	636
INCOME (LOSS) FROM CONTINUING OPERATIONS BEFORE INCOME TAXES	311	336
Allocated income taxes and TE adjustments	88	101
Income (loss) from continuing operations	223	235
Net income (loss)	223	235
Less: Net income (loss) attributable to noncontrolling interests	2
Net income (loss) attributable to Key	221	235
AVERAGE BALANCES
Loans and leases	57,428	54,701
Total assets	88,951	84,997
Deposits	69,450	66,478
OTHER FINANCIAL DATA
Net loan charge-offs	28	20
Return on average allocated equity	17.84%	19.59%
Return on average allocated equity	17.84%	19.59%
Average full-time equivalent employees	9,555	9,686
Operating Segments [Member] | Key Community Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	358	363
Noninterest income	191	183
Total revenue (TE)	549	546
Provision (credit) for loan and lease losses	29	11
Depreciation and amortization expense	15	17
Other noninterest expense	425	419
INCOME (LOSS) FROM CONTINUING OPERATIONS BEFORE INCOME TAXES	80	99
Allocated income taxes and TE adjustments	30	37
Income (loss) from continuing operations	50	62
Net income (loss)	50	62
Net income (loss) attributable to Key	50	62
AVERAGE BALANCES
Loans and leases	30,662	29,797
Total assets	32,716	31,918
Deposits	50,417	49,910
OTHER FINANCIAL DATA
Net loan charge-offs	28	28
Return on average allocated equity	7.38%	8.97%
Return on average allocated equity	7.38%	8.97%
Average full-time equivalent employees	7,475	7,698
Operating Segments [Member] | Key Corporate Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	213	196
Noninterest income	188	196
Total revenue (TE)	401	392
Provision (credit) for loan and lease losses	8	(3)
Depreciation and amortization expense	10	7
Other noninterest expense	207	195
INCOME (LOSS) FROM CONTINUING OPERATIONS BEFORE INCOME TAXES	176	193
Allocated income taxes and TE adjustments	49	57
Income (loss) from continuing operations	127	136
Net income (loss)	127	136
Less: Net income (loss) attributable to noncontrolling interests	1
Net income (loss) attributable to Key	126	136
AVERAGE BALANCES
Loans and leases	24,722	21,991
Total assets	30,297	27,171
Deposits	18,567	15,993
OTHER FINANCIAL DATA
Net loan charge-offs	(4)	(12)
Return on average allocated equity	27.44%	35.65%
Return on average allocated equity	27.44%	35.65%
Average full-time equivalent employees	2,064	1,916
Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	4	10
Noninterest income	63	55
Total revenue (TE)	67	65
Provision (credit) for loan and lease losses	(3)	(4)
Depreciation and amortization expense	2	3
Other noninterest expense	13	22
INCOME (LOSS) FROM CONTINUING OPERATIONS BEFORE INCOME TAXES	55	44
Allocated income taxes and TE adjustments	9	7
Income (loss) from continuing operations	46	37
Net income (loss)	46	37
Less: Net income (loss) attributable to noncontrolling interests	1
Net income (loss) attributable to Key	45	37
AVERAGE BALANCES
Loans and leases	2,044	2,913
Total assets	25,938	25,908
Deposits	466	575
OTHER FINANCIAL DATA
Net loan charge-offs	4	4
Return on average allocated equity	43.98%	29.14%
Return on average allocated equity	43.98%	29.14%
Average full-time equivalent employees	16	72
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	2
Noninterest income	(5)	1
Total revenue (TE)	(3)	1
Provision (credit) for loan and lease losses	1	2
Depreciation and amortization expense	37	38
Other noninterest expense	(40)	(39)
INCOME (LOSS) FROM CONTINUING OPERATIONS BEFORE INCOME TAXES	(1)
Allocated income taxes and TE adjustments	(8)	(3)
Income (loss) from continuing operations	7	3
Income (loss) from discontinued operations, net of taxes	5	4
Net income (loss)	12	7
Net income (loss) attributable to Key	12	7
AVERAGE BALANCES
Loans and leases	84	45
Total assets	678	660
Deposits	(81)	(184)
OTHER FINANCIAL DATA
Return on average allocated equity	0.51%	0.22%
Return on average allocated equity	0.88%	0.52%
Average full-time equivalent employees	4,036	4,369
Operating Segments and Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	577	569
Noninterest income	437	435
Total revenue (TE)	1,014	1,004
Provision (credit) for loan and lease losses	35	6
Depreciation and amortization expense	64	65
Other noninterest expense	605	597
INCOME (LOSS) FROM CONTINUING OPERATIONS BEFORE INCOME TAXES	310	336
Allocated income taxes and TE adjustments	80	98
Income (loss) from continuing operations	230	238
Income (loss) from discontinued operations, net of taxes	5	4
Net income (loss)	235	242
Less: Net income (loss) attributable to noncontrolling interests	2
Net income (loss) attributable to Key	233	242
AVERAGE BALANCES
Loans and leases	57,512	54,746
Total assets	89,629	85,657
Deposits	69,369	66,294
OTHER FINANCIAL DATA
Net loan charge-offs	$ 28	$ 20
Return on average allocated equity	8.75%	9.31%
Return on average allocated equity	8.94%	9.46%
Average full-time equivalent employees	13,591	14,055