Line of Business Results - Financial Information of Business Groups (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) employee	Sep. 30, 2017 USD ($) employee	Sep. 30, 2018 USD ($) employee	Sep. 30, 2017 USD ($) employee
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 993	$ 962	$ 2,932	$ 2,878
Noninterest income	609	592	1,870	1,822
Total revenue (TE)	1,602	1,554	4,802	4,700
Provision for credit losses	62	51	187	180
Depreciation and amortization expense	98	97	300	283
Other noninterest expense	866	895	2,663	2,717
Income (loss) from continuing operations before income taxes	576	511	1,652	1,520
Allocated income taxes and TE adjustments	94	148	275	425
INCOME (LOSS) FROM CONTINUING OPERATIONS	482	363	1,377	1,095
Income (loss) from discontinued operations, net of taxes	0	1	5	6
NET INCOME (LOSS)	482	364	1,382	1,101
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	1
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	482	364	1,382	1,100
AVERAGE BALANCES
Loans and leases	88,467	86,814	88,018	86,485
Total assets	136,729	134,356	136,014	133,202
Deposits	105,626	103,105	104,073	102,659
OTHER FINANCIAL DATA
Net loan charge-offs	$ 60	$ 32	$ 174	$ 156
Return on average allocated equity	12.57%	9.45%	12.24%	9.62%
Return on average allocated equity	12.57%	9.48%	12.28%	9.67%
Average full-time equivalent employees | employee	18,150	18,548	18,354	18,427
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 986	$ 955	$ 2,911	$ 2,865
Noninterest income	606	593	1,811	1,837
Total revenue (TE)	1,592	1,548	4,722	4,702
Provision for credit losses	61	50	186	180
Depreciation and amortization expense	61	56	186	156
Other noninterest expense	897	899	2,743	2,701
Income (loss) from continuing operations before income taxes	573	543	1,607	1,665
Allocated income taxes and TE adjustments	111	170	287	497
INCOME (LOSS) FROM CONTINUING OPERATIONS	462	373	1,320	1,168
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	462	373	1,320	1,168
Less: Net income (loss) attributable to noncontrolling interests	0	(1)	0	1
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	462	374	1,320	1,167
AVERAGE BALANCES
Loans and leases	88,464	86,810	88,014	86,463
Total assets	136,369	133,794	135,628	132,436
Deposits	105,607	103,132	104,022	102,706
OTHER FINANCIAL DATA
Net loan charge-offs	$ 62	$ 32	$ 175	$ 156
Return on average allocated equity	23.26%	19.11%	22.39%	20.03%
Return on average allocated equity	23.26%	19.11%	22.39%	20.03%
Average full-time equivalent employees | employee	13,077	13,493	13,319	13,317
Operating Segments [Member] | Key Community Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 726	$ 673	$ 2,129	$ 1,978
Noninterest income	268	272	820	856
Total revenue (TE)	994	945	2,949	2,834
Provision for credit losses	43	59	128	152
Depreciation and amortization expense	27	30	84	86
Other noninterest expense	608	596	1,844	1,790
Income (loss) from continuing operations before income taxes	316	260	893	806
Allocated income taxes and TE adjustments	75	97	212	300
INCOME (LOSS) FROM CONTINUING OPERATIONS	241	163	681	506
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	241	163	681	506
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	241	163	681	506
AVERAGE BALANCES
Loans and leases	47,862	47,614	47,844	47,396
Total assets	51,740	51,642	51,739	51,361
Deposits	82,259	79,563	81,053	79,439
OTHER FINANCIAL DATA
Net loan charge-offs	$ 43	$ 41	$ 119	$ 131
Return on average allocated equity	19.80%	13.39%	18.80%	14.14%
Return on average allocated equity	19.80%	13.39%	18.80%	14.14%
Average full-time equivalent employees | employee	10,529	10,696	10,604	10,573
Operating Segments [Member] | Key Corporate Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 273	$ 292	$ 822	$ 909
Noninterest income	301	269	852	827
Total revenue (TE)	574	561	1,674	1,736
Provision for credit losses	20	(11)	62	26
Depreciation and amortization expense	34	25	101	68
Other noninterest expense	282	278	852	834
Income (loss) from continuing operations before income taxes	238	269	659	808
Allocated income taxes and TE adjustments	39	79	85	213
INCOME (LOSS) FROM CONTINUING OPERATIONS	199	190	574	595
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	199	190	574	595
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	199	190	574	595
AVERAGE BALANCES
Loans and leases	39,714	38,021	39,232	37,804
Total assets	46,860	45,257	46,544	44,506
Deposits	21,056	21,559	20,977	21,237
OTHER FINANCIAL DATA
Net loan charge-offs	$ 19	$ (9)	$ 56	$ 24
Return on average allocated equity	26.91%	27.08%	26.36%	27.93%
Return on average allocated equity	26.91%	27.08%	26.36%	27.93%
Average full-time equivalent employees | employee	2,546	2,460	2,542	2,403
Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ (13)	$ (10)	$ (40)	$ (22)
Noninterest income	37	52	139	154
Total revenue (TE)	24	42	99	132
Provision for credit losses	(2)	2	(4)	2
Depreciation and amortization expense	0	1	1	2
Other noninterest expense	7	25	47	77
Income (loss) from continuing operations before income taxes	19	14	55	51
Allocated income taxes and TE adjustments	(3)	(6)	(10)	(16)
INCOME (LOSS) FROM CONTINUING OPERATIONS	22	20	65	67
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	22	20	65	67
Less: Net income (loss) attributable to noncontrolling interests	0	(1)	0	1
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	22	21	65	66
AVERAGE BALANCES
Loans and leases	888	1,175	938	1,263
Total assets	37,769	36,895	37,345	36,569
Deposits	2,292	2,010	1,992	2,030
OTHER FINANCIAL DATA
Net loan charge-offs	$ (1)	$ 0	$ 0	$ 1
Return on average allocated equity	73.35%	55.54%	67.37%	56.20%
Return on average allocated equity	73.35%	55.54%	67.37%	56.20%
Average full-time equivalent employees | employee	2	337	173	341
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 7	$ 7	$ 21	$ 13
Noninterest income	3	(1)	59	(15)
Total revenue (TE)	10	6	80	(2)
Provision for credit losses	1	1	1	0
Depreciation and amortization expense	37	41	114	127
Other noninterest expense	(31)	(4)	(80)	16
Income (loss) from continuing operations before income taxes	3	(32)	45	(145)
Allocated income taxes and TE adjustments	(17)	(22)	(12)	(72)
INCOME (LOSS) FROM CONTINUING OPERATIONS	20	(10)	57	(73)
Income (loss) from discontinued operations, net of taxes	0	1	5	6
NET INCOME (LOSS)	20	(9)	62	(67)
Less: Net income (loss) attributable to noncontrolling interests	0	1	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	20	(10)	62	(67)
AVERAGE BALANCES
Loans and leases	3	4	4	22
Total assets	360	562	386	766
Deposits	19	(27)	51	(47)
OTHER FINANCIAL DATA
Net loan charge-offs	$ (1)	$ 0	$ (1)	$ 0
Return on average allocated equity	1.08%	(0.58%)	1.06%	(1.32%)
Return on average allocated equity	1.08%	(0.53%)	1.16%	(1.21%)
Average full-time equivalent employees | employee	5,073	5,055	5,035	5,110