Line of Business Results - Financial Information of Business Groups (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) Employees	Sep. 30, 2014 USD ($) Employees	Sep. 30, 2015 USD ($) Employees	Sep. 30, 2014 USD ($) Employees
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 591	$ 575	$ 1,746	$ 1,711
Noninterest income	470	417	1,395	1,307
Provision for credit losses			110	52
Other noninterest expense	88	89	258	251
Allocated income taxes and TE adjustments	72	64	230	232
Income (loss) from continuing operations	220	203	686	694
Income (loss) from discontinued operations, net of taxes	(3)	(17)	5	(41)
Net income (loss)	217	186	691	653
Less: Net income (loss) attributable to noncontrolling interests	(2)		1	6
Net income (loss) attributable to Key	219	186	690	647
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	595	580	1,759	1,726
Noninterest income	473	422	1,406	1,314
Total revenue (TE)	1,068	1,002	3,165	3,040
Provision for credit losses	44	19	120	36
Depreciation and amortization expense	27	27	79	80
Other noninterest expense	682	647	2,016	1,927
Income (loss) from continuing operations before income taxes (TE)	315	309	950	997
Allocated income taxes and TE adjustments	82	88	261	292
Income (loss) from continuing operations	233	221	689	705
Net income (loss)	233	221	689	705
Less: Net income (loss) attributable to noncontrolling interests	(2)		1	6
Net income (loss) attributable to Key	235	221	688	699
AVERAGE BALANCES
Loans and leases	59,244	55,714	58,200	55,320
Total assets	92,124	86,458	90,706	85,836
Deposits	70,498	68,466	70,291	67,397
OTHER FINANCIAL DATA
Net loan charge-offs	$ 42	$ 31	$ 105	$ 82
Return on average allocated equity	19.00%	18.28%	18.81%	19.66%
Return on average allocated equity	19.00%	18.28%	18.81%	19.66%
Operating Segments [Member] | Weighted Average [Member]
OTHER FINANCIAL DATA
Average full-time equivalent employees | Employees	9,513	9,605	9,503	9,615
Operating Segments [Member] | Key Community Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 379	$ 359	$ 1,098	$ 1,084
Noninterest income	200	199	589	574
Total revenue (TE)	579	558	1,687	1,658
Provision for credit losses	18	21	50	48
Depreciation and amortization expense	14	16	42	49
Other noninterest expense	434	425	1,297	1,274
Income (loss) from continuing operations before income taxes (TE)	113	96	298	287
Allocated income taxes and TE adjustments	42	36	111	107
Income (loss) from continuing operations	71	60	187	180
Net income (loss)	71	60	187	180
Net income (loss) attributable to Key	71	60	187	180
AVERAGE BALANCES
Loans and leases	31,039	30,103	30,804	29,979
Total assets	33,090	32,173	32,853	32,063
Deposits	51,234	50,303	50,808	50,151
OTHER FINANCIAL DATA
Net loan charge-offs	$ 21	$ 28	$ 69	$ 89
Return on average allocated equity	10.49%	8.89%	9.27%	8.81%
Return on average allocated equity	10.49%	8.89%	9.27%	8.81%
Operating Segments [Member] | Key Community Bank [Member] | Weighted Average [Member]
OTHER FINANCIAL DATA
Average full-time equivalent employees | Employees	7,326	7,573	7,392	7,613
Operating Segments [Member] | Key Corporate Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 220	$ 215	$ 661	$ 621
Noninterest income	234	185	671	566
Total revenue (TE)	454	400	1,332	1,187
Provision for credit losses	30	2	77	7
Depreciation and amortization expense	11	8	31	22
Other noninterest expense	235	205	681	599
Income (loss) from continuing operations before income taxes (TE)	178	185	543	559
Allocated income taxes and TE adjustments	42	51	143	158
Income (loss) from continuing operations	136	134	400	401
Net income (loss)	136	134	400	401
Less: Net income (loss) attributable to noncontrolling interests	(2)		(1)	2
Net income (loss) attributable to Key	138	134	401	399
AVERAGE BALANCES
Loans and leases	26,425	23,215	25,488	22,702
Total assets	32,163	28,268	31,236	27,829
Deposits	18,809	17,599	19,029	16,655
OTHER FINANCIAL DATA
Net loan charge-offs	$ 20	$ (1)	$ 28	$ (15)
Return on average allocated equity	28.65%	31.59%	28.82%	33.74%
Return on average allocated equity	28.65%	31.59%	28.82%	33.74%
Operating Segments [Member] | Key Corporate Bank [Member] | Weighted Average [Member]
OTHER FINANCIAL DATA
Average full-time equivalent employees | Employees	2,173	1,998	2,096	1,952
Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ (4)	$ 6		$ 21
Noninterest income	39	38	$ 146	174
Total revenue (TE)	35	44	146	195
Provision for credit losses	(4)	(4)	(7)	(19)
Depreciation and amortization expense	2	3	6	9
Other noninterest expense	13	17	38	54
Income (loss) from continuing operations before income taxes (TE)	24	28	109	151
Allocated income taxes and TE adjustments	(2)	1	7	27
Income (loss) from continuing operations	26	27	102	124
Net income (loss)	26	27	102	124
Less: Net income (loss) attributable to noncontrolling interests			2	4
Net income (loss) attributable to Key	26	27	100	120
AVERAGE BALANCES
Loans and leases	1,780	2,396	1,908	2,639
Total assets	26,871	26,017	26,617	25,944
Deposits	455	564	454	591
OTHER FINANCIAL DATA
Net loan charge-offs	$ 1	$ 4	$ 8	$ 8
Return on average allocated equity	33.27%	24.63%	40.39%	36.38%
Return on average allocated equity	33.27%	24.63%	40.39%	36.38%
Operating Segments [Member] | Other Segments [Member] | Weighted Average [Member]
OTHER FINANCIAL DATA
Average full-time equivalent employees | Employees	14	34	15	50
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 3	$ 1	$ 7	$ 3
Noninterest income	(3)	(5)	(11)	(7)
Total revenue (TE)		(4)	(4)	(4)
Provision for credit losses	1		1	(1)
Depreciation and amortization expense	35	38	112	115
Other noninterest expense	(20)	(6)	(103)	(65)
Income (loss) from continuing operations before income taxes (TE)	(16)	(36)	(14)	(53)
Allocated income taxes and TE adjustments	(3)	(18)	(11)	(42)
Income (loss) from continuing operations	(13)	(18)	(3)	(11)
Income (loss) from discontinued operations, net of taxes	(3)	(17)	5	(41)
Net income (loss)	(16)	(35)	2	(52)
Net income (loss) attributable to Key	(16)	(35)	2	(52)
AVERAGE BALANCES
Loans and leases	37	82	63	68
Total assets	527	665	618	676
Deposits	(29)	$ (130)	$ (62)	(138)
OTHER FINANCIAL DATA
Net loan charge-offs	$ (1)			$ (1)
Return on average allocated equity	(0.90%)	(1.26%)	(0.05%)	(0.26%)
Return on average allocated equity	(1.11%)	(2.45%)	0.07%	(1.22%)
Segment Reconciling Items [Member] | Weighted Average [Member]
OTHER FINANCIAL DATA
Average full-time equivalent employees | Employees	4,042	4,300	4,022	4,327
Operating Segments and Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 598	$ 581	$ 1,766	$ 1,729
Noninterest income	470	417	1,395	1,307
Total revenue (TE)	1,068	998	3,161	3,036
Provision for credit losses	45	19	121	35
Depreciation and amortization expense	62	65	191	195
Other noninterest expense	662	641	1,913	1,862
Income (loss) from continuing operations before income taxes (TE)	299	273	936	944
Allocated income taxes and TE adjustments	79	70	250	250
Income (loss) from continuing operations	220	203	686	694
Income (loss) from discontinued operations, net of taxes	(3)	(17)	5	(41)
Net income (loss)	217	186	691	653
Less: Net income (loss) attributable to noncontrolling interests	(2)		1	6
Net income (loss) attributable to Key	219	186	690	647
AVERAGE BALANCES
Loans and leases	59,281	55,796	58,263	55,388
Total assets	92,651	87,123	91,324	86,512
Deposits	70,469	68,336	70,229	67,259
OTHER FINANCIAL DATA
Net loan charge-offs	$ 41	$ 31	$ 105	$ 81
Return on average allocated equity	8.30%	7.69%	6.48%	6.59%
Return on average allocated equity	8.19%	7.05%	6.52%	6.20%
Operating Segments and Reconciling Items [Member] | Weighted Average [Member]
OTHER FINANCIAL DATA
Average full-time equivalent employees | Employees	13,555	13,905	13,525	13,942