Line of Business Results - Financial Information of Business Groups (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) employee	Jun. 30, 2017 USD ($) employee	Jun. 30, 2018 USD ($) employee	Jun. 30, 2017 USD ($) employee
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 987	$ 987	$ 1,939	$ 1,916
Noninterest income	660	653	1,261	1,230
Total revenue (TE)	1,647	1,640	3,200	3,146
Provision for credit losses	64	66	125	129
Depreciation and amortization expense	99	93	203	186
Other noninterest expense	894	902	1,796	1,822
Income (loss) from continuing operations before income taxes	590	579	1,076	1,009
Allocated income taxes and TE adjustments	111	172	181	277
INCOME (LOSS) FROM CONTINUING OPERATIONS	479	407	895	732
Income (loss) from discontinued operations, net of taxes	3	5	5	5
NET INCOME (LOSS)	482	412	900	737
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	1
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	482	412	900	736
AVERAGE BALANCES
Loans and leases	88,644	86,502	87,790	86,318
Total assets	136,378	132,491	135,651	132,615
Deposits	104,003	102,779	103,283	102,430
OTHER FINANCIAL DATA
Net loan charge-offs	$ 60	$ 66	$ 114	$ 124
Return on average allocated equity	12.78%	10.74%	12.06%	9.70%
Return on average allocated equity	12.86%	10.87%	12.13%	9.77%
Average full-time equivalent employees | employee	18,376	18,344	18,458	18,365
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 980	$ 980	$ 1,926	$ 1,909
Noninterest income	596	661	1,204	1,244
Total revenue (TE)	1,576	1,641	3,130	3,153
Provision for credit losses	65	66	126	130
Depreciation and amortization expense	61	51	125	99
Other noninterest expense	920	908	1,845	1,798
Income (loss) from continuing operations before income taxes	530	616	1,034	1,126
Allocated income taxes and TE adjustments	94	170	175	329
INCOME (LOSS) FROM CONTINUING OPERATIONS	436	446	859	797
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	436	446	859	797
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	1
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	436	446	859	796
AVERAGE BALANCES
Loans and leases	88,628	86,469	87,785	86,286
Total assets	136,011	131,753	135,252	131,746
Deposits	103,947	102,796	103,217	102,489
OTHER FINANCIAL DATA
Net loan charge-offs	$ 60	$ 66	$ 114	$ 124
Return on average allocated equity	22.22%	22.99%	22.10%	20.57%
Return on average allocated equity	22.22%	22.99%	22.10%	20.57%
Average full-time equivalent employees | employee	13,351	13,264	13,441	13,228
Operating Segments [Member] | Key Community Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 715	$ 676	$ 1,403	$ 1,305
Noninterest income	281	322	551	583
Total revenue (TE)	996	998	1,954	1,888
Provision for credit losses	38	47	86	94
Depreciation and amortization expense	28	28	57	56
Other noninterest expense	611	607	1,233	1,185
Income (loss) from continuing operations before income taxes	319	316	578	553
Allocated income taxes and TE adjustments	75	118	137	206
INCOME (LOSS) FROM CONTINUING OPERATIONS	244	198	441	347
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	244	198	441	347
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	244	198	441	347
AVERAGE BALANCES
Loans and leases	47,984	47,477	47,833	47,282
Total assets	51,866	51,441	51,736	51,215
Deposits	80,930	79,601	80,440	79,375
OTHER FINANCIAL DATA
Net loan charge-offs	$ 34	$ 47	$ 76	$ 90
Return on average allocated equity	20.22%	16.59%	18.43%	14.70%
Return on average allocated equity	20.22%	16.59%	18.43%	14.70%
Average full-time equivalent employees | employee	10,619	10,558	10,642	10,511
Operating Segments [Member] | Key Corporate Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 277	$ 312	$ 549	$ 616
Noninterest income	265	285	552	559
Total revenue (TE)	542	597	1,101	1,175
Provision for credit losses	28	19	42	36
Depreciation and amortization expense	33	22	67	42
Other noninterest expense	293	275	573	561
Income (loss) from continuing operations before income taxes	188	281	419	536
Allocated income taxes and TE adjustments	21	57	45	133
INCOME (LOSS) FROM CONTINUING OPERATIONS	167	224	374	403
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	167	224	374	403
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	(1)
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	167	224	374	404
AVERAGE BALANCES
Loans and leases	39,710	37,704	38,989	37,696
Total assets	47,213	44,131	46,386	44,128
Deposits	21,057	21,145	20,937	21,074
OTHER FINANCIAL DATA
Net loan charge-offs	$ 26	$ 19	$ 38	$ 33
Return on average allocated equity	23.07%	31.66%	26.21%	28.27%
Return on average allocated equity	23.07%	31.66%	26.21%	28.27%
Average full-time equivalent employees | employee	2,537	2,364	2,540	2,374
Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ (12)	$ (8)	$ (26)	$ (12)
Noninterest income	50	54	101	102
Total revenue (TE)	38	46	75	90
Provision for credit losses	(1)	0	(2)	0
Depreciation and amortization expense	0	1	1	1
Other noninterest expense	16	26	39	52
Income (loss) from continuing operations before income taxes	23	19	37	37
Allocated income taxes and TE adjustments	(2)	(5)	(7)	(10)
INCOME (LOSS) FROM CONTINUING OPERATIONS	25	24	44	47
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	25	24	44	47
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	2
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	25	24	44	45
AVERAGE BALANCES
Loans and leases	934	1,288	963	1,308
Total assets	36,932	36,181	37,130	36,403
Deposits	1,960	2,050	1,840	2,040
OTHER FINANCIAL DATA
Net loan charge-offs	$ 1	$ 0	$ 1	$ 1
Return on average allocated equity	78.96%	61.31%	66.22%	56.36%
Return on average allocated equity	78.96%	61.31%	66.22%	56.36%
Average full-time equivalent employees | employee	195	342	259	343
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 7	$ 7	$ 13	$ 7
Noninterest income	64	(8)	57	(14)
Total revenue (TE)	71	(1)	70	(7)
Provision for credit losses	(1)	0	(1)	(1)
Depreciation and amortization expense	38	42	78	87
Other noninterest expense	(26)	(6)	(49)	24
Income (loss) from continuing operations before income taxes	60	(37)	42	(117)
Allocated income taxes and TE adjustments	17	2	6	(52)
INCOME (LOSS) FROM CONTINUING OPERATIONS	43	(39)	36	(65)
Income (loss) from discontinued operations, net of taxes	3	5	5	5
NET INCOME (LOSS)	46	(34)	41	(60)
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	46	(34)	41	(60)
AVERAGE BALANCES
Loans and leases	16	33	5	32
Total assets	367	738	399	869
Deposits	56	(17)	66	(59)
OTHER FINANCIAL DATA
Net loan charge-offs	$ 0	$ 0	$ 0	$ 0
Return on average allocated equity	2.41%	(2.11%)	1.02%	(1.77%)
Return on average allocated equity	2.58%	(1.84%)	1.16%	(1.64%)
Average full-time equivalent employees | employee	5,025	5,080	5,017	5,137