Line of Business Results - Financial Information of Business Groups (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) employee	Dec. 31, 2017 USD ($) employee	Dec. 31, 2016 USD ($) employee
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 3,940	$ 3,830	$ 2,953
Noninterest income	2,515	2,478	2,071
Total revenue (TE)	6,455	6,308	5,024
Provision for credit losses	246	229	266
Depreciation and amortization expense	400	384	303
Other noninterest expense	3,575	3,714	3,453
Income (loss) from continuing operations before income taxes (TE)	2,234	1,981	1,002
Allocated income taxes (benefit) and TE adjustments	375	690	213
INCOME (LOSS) FROM CONTINUING OPERATIONS	1,859	1,291	789
Income (loss) from discontinued operations, net of taxes	7	7	1
NET INCOME (LOSS)	1,866	1,298	790
Less: Net income (loss) attributable to noncontrolling interests	0	2	(1)
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	1,866	1,296	791
AVERAGE BALANCES
Loans and leases	88,338	86,365	71,148
Total assets	136,812	133,719	112,537
Deposits	105,051	102,946	86,352
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	3,001	3,097	1,934
Net loan charge-offs	$ 234	$ 208	$ 205
Return on average allocated equity, continuing operations (as a percentage)	12.29%	8.47%	6.25%
Return on average allocated equity (as a percentage)	12.33%	8.51%	6.25%
Average full-time equivalent employees | employee	18,180	18,415	15,700
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 3,912	$ 3,810	$ 2,957
Noninterest income	2,465	2,499	2,089
Total revenue (TE)	6,377	6,309	5,046
Provision for credit losses	246	229	266
Depreciation and amortization expense	246	215	140
Other noninterest expense	3,651	3,683	3,182
Income (loss) from continuing operations before income taxes (TE)	2,234	2,182	1,458
Allocated income taxes (benefit) and TE adjustments	393	589	376
INCOME (LOSS) FROM CONTINUING OPERATIONS	1,841	1,593	1,082
Income (loss) from discontinued operations, net of taxes	0	0	0
NET INCOME (LOSS)	1,841	1,593	1,082
Less: Net income (loss) attributable to noncontrolling interests	0	3	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	1,841	1,590	1,082
AVERAGE BALANCES
Loans and leases	88,329	86,340	71,035
Total assets	136,451	133,033	110,035
Deposits	105,007	102,975	85,868
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	2,900	3,016	1,818
Net loan charge-offs	$ 233	$ 207	$ 204
Return on average allocated equity, continuing operations (as a percentage)	23.37%	20.41%	18.36%
Return on average allocated equity (as a percentage)	23.37%	20.41%	18.36%
Average full-time equivalent employees | employee	13,159	13,332	11,185
Operating Segments [Member] | Key Community Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 2,873	$ 2,652	$ 1,953
Noninterest income	1,098	1,143	906
Total revenue (TE)	3,971	3,795	2,859
Provision for credit losses	177	209	143
Depreciation and amortization expense	110	116	76
Other noninterest expense	2,451	2,424	2,048
Income (loss) from continuing operations before income taxes (TE)	1,233	1,046	592
Allocated income taxes (benefit) and TE adjustments	291	388	220
INCOME (LOSS) FROM CONTINUING OPERATIONS	942	658	372
Income (loss) from discontinued operations, net of taxes	0	0	0
NET INCOME (LOSS)	942	658	372
Less: Net income (loss) attributable to noncontrolling interests	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	942	658	372
AVERAGE BALANCES
Loans and leases	47,877	47,399	37,624
Total assets	51,774	51,370	40,300
Deposits	81,868	79,669	63,875
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	2,352	2,438	1,478
Net loan charge-offs	$ 161	$ 166	$ 114
Return on average allocated equity, continuing operations (as a percentage)	19.50%	13.71%	10.96%
Return on average allocated equity (as a percentage)	19.50%	13.71%	10.96%
Average full-time equivalent employees | employee	10,501	10,587	8,794
Operating Segments [Member] | Key Corporate Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 1,094	$ 1,193	$ 1,049
Noninterest income	1,161	1,148	1,013
Total revenue (TE)	2,255	2,341	2,062
Provision for credit losses	74	20	127
Depreciation and amortization expense	135	96	60
Other noninterest expense	1,147	1,158	1,073
Income (loss) from continuing operations before income taxes (TE)	899	1,067	802
Allocated income taxes (benefit) and TE adjustments	110	249	178
INCOME (LOSS) FROM CONTINUING OPERATIONS	789	818	624
Income (loss) from discontinued operations, net of taxes	0	0	0
NET INCOME (LOSS)	789	818	624
Less: Net income (loss) attributable to noncontrolling interests	0	0	(2)
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	789	818	626
AVERAGE BALANCES
Loans and leases	39,536	37,716	31,925
Total assets	47,126	44,505	37,797
Deposits	21,183	21,318	20,780
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	538	559	340
Net loan charge-offs	$ 73	$ 40	$ 83
Return on average allocated equity, continuing operations (as a percentage)	27.01%	28.82%	26.89%
Return on average allocated equity (as a percentage)	27.01%	28.82%	26.89%
Average full-time equivalent employees | employee	2,528	2,407	2,244
Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ (55)	$ (35)	$ (45)
Noninterest income	206	208	170
Total revenue (TE)	151	173	125
Provision for credit losses	(5)	0	(4)
Depreciation and amortization expense	1	3	4
Other noninterest expense	53	101	61
Income (loss) from continuing operations before income taxes (TE)	102	69	64
Allocated income taxes (benefit) and TE adjustments	(8)	(48)	(22)
INCOME (LOSS) FROM CONTINUING OPERATIONS	110	117	86
Income (loss) from discontinued operations, net of taxes	0	0	0
NET INCOME (LOSS)	110	117	86
Less: Net income (loss) attributable to noncontrolling interests	0	3	2
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	110	114	84
AVERAGE BALANCES
Loans and leases	916	1,225	1,486
Total assets	37,551	37,158	31,938
Deposits	1,956	1,988	1,213
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	10	19	0
Net loan charge-offs	$ (1)	$ 1	$ 7
Return on average allocated equity, continuing operations (as a percentage)	88.00%	73.55%	48.84%
Return on average allocated equity (as a percentage)	88.00%	73.55%	48.84%
Average full-time equivalent employees | employee	130	338	147
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 28	$ 20	$ (4)
Noninterest income	50	(21)	(18)
Total revenue (TE)	78	(1)	(22)
Provision for credit losses	0	0	0
Depreciation and amortization expense	154	169	163
Other noninterest expense	(76)	31	271
Income (loss) from continuing operations before income taxes (TE)	0	(201)	(456)
Allocated income taxes (benefit) and TE adjustments	(18)	101	(163)
INCOME (LOSS) FROM CONTINUING OPERATIONS	18	(302)	(293)
Income (loss) from discontinued operations, net of taxes	7	7	1
NET INCOME (LOSS)	25	(295)	(292)
Less: Net income (loss) attributable to noncontrolling interests	0	(1)	(1)
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	25	(294)	(291)
AVERAGE BALANCES
Loans and leases	9	25	113
Total assets	361	686	2,502
Deposits	44	(29)	484
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	101	81	116
Net loan charge-offs	$ 1	$ 1	$ 1
Return on average allocated equity, continuing operations (as a percentage)	0.25%	(4.05%)	(4.32%)
Return on average allocated equity (as a percentage)	0.34%	(3.96%)	(4.31%)
Average full-time equivalent employees | employee	5,021	5,083	4,515