Business Segment Reporting - Financial Information of Business Groups (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) employee	Dec. 31, 2018 USD ($) employee	Dec. 31, 2017 USD ($) employee
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 3,941	$ 3,940	$ 3,830
Noninterest income	2,459	2,515	2,478
Total revenue (TE)	6,400	6,455	6,308
Provision for credit losses	445	246	229
Depreciation and amortization expense	374	400	384
Other noninterest expense	3,527	3,575	3,714
Income (loss) from continuing operations before income taxes (TE)	2,054	2,234	1,981
Allocated income taxes (benefit) and TE adjustments	346	375	690
INCOME (LOSS) FROM CONTINUING OPERATIONS	1,708	1,859	1,291
Income (loss) from discontinued operations, net of taxes	9	7	7
NET INCOME (LOSS)	1,717	1,866	1,298
Less: Net income (loss) attributable to noncontrolling interests	0	0	2
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	1,717	1,866	1,296
AVERAGE BALANCES
Loans and leases	91,511	88,338	86,365
Total assets	143,179	136,812	133,719
Deposits	110,030	105,051	102,946
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	245	48	267
Net loan charge-offs	$ 424	$ 234	$ 208
Return on average allocated equity, continuing operations (as a percentage)	10.27%	12.29%	8.47%
Return on average allocated equity (as a percentage)	10.32%	12.33%	8.51%
Average full-time equivalent employees | employee	17,045	18,180	18,415
Provision for credit loss, net of tax, related to previously disclosed fraud incident	$ 139
Operating Segments [Member] | Consumer Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	2,366	$ 2,306	$ 2,133
Noninterest income	922	915	976
Total revenue (TE)	3,288	3,221	3,109
Provision for credit losses	188	147	145
Depreciation and amortization expense	97	103	108
Other noninterest expense	2,078	2,143	2,167
Income (loss) from continuing operations before income taxes (TE)	925	828	689
Allocated income taxes (benefit) and TE adjustments	219	196	255
INCOME (LOSS) FROM CONTINUING OPERATIONS	706	632	434
Income (loss) from discontinued operations, net of taxes	0	0	0
NET INCOME (LOSS)	706	632	434
Less: Net income (loss) attributable to noncontrolling interests	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	706	632	434
AVERAGE BALANCES
Loans and leases	32,536	31,307	32,092
Total assets	36,096	34,523	35,360
Deposits	72,544	68,821	66,711
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	150	(38)	90
Net loan charge-offs	$ 157	$ 149	$ 152
Return on average allocated equity, continuing operations (as a percentage)	21.30%	19.24%	13.19%
Return on average allocated equity (as a percentage)	21.30%	19.24%	13.19%
Average full-time equivalent employees | employee	9,292	9,957	9,990
Operating Segments [Member] | Commercial Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 1,621	$ 1,657	$ 1,708
Noninterest income	1,390	1,328	1,272
Total revenue (TE)	3,011	2,985	2,980
Provision for credit losses	118	102	84
Depreciation and amortization expense	135	139	103
Other noninterest expense	1,388	1,430	1,393
Income (loss) from continuing operations before income taxes (TE)	1,370	1,314	1,400
Allocated income taxes (benefit) and TE adjustments	223	207	373
INCOME (LOSS) FROM CONTINUING OPERATIONS	1,147	1,107	1,027
Income (loss) from discontinued operations, net of taxes	0	0	0
NET INCOME (LOSS)	1,147	1,107	1,027
Less: Net income (loss) attributable to noncontrolling interests	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	1,147	1,107	1,027
AVERAGE BALANCES
Loans and leases	57,988	55,828	52,783
Total assets	66,122	63,684	59,956
Deposits	36,212	33,675	33,781
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	(8)	(17)	69
Net loan charge-offs	$ 128	$ 85	$ 55
Return on average allocated equity, continuing operations (as a percentage)	24.99%	24.94%	23.91%
Return on average allocated equity (as a percentage)	24.99%	24.94%	23.91%
Average full-time equivalent employees | employee	2,232	2,449	2,331
Corporate And Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ (46)	$ (23)	$ (11)
Noninterest income	147	272	230
Total revenue (TE)	101	249	219
Provision for credit losses	139	(3)	0
Depreciation and amortization expense	142	158	173
Other noninterest expense	61	2	154
Income (loss) from continuing operations before income taxes (TE)	(241)	92	(108)
Allocated income taxes (benefit) and TE adjustments	(96)	(28)	62
INCOME (LOSS) FROM CONTINUING OPERATIONS	(145)	120	(170)
Income (loss) from discontinued operations, net of taxes	9	7	7
NET INCOME (LOSS)	(136)	127	(163)
Less: Net income (loss) attributable to noncontrolling interests	0	0	2
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	(136)	127	(165)
AVERAGE BALANCES
Loans and leases	987	1,203	1,490
Total assets	40,961	38,605	38,403
Deposits	1,274	2,555	2,454
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	103	103	108
Net loan charge-offs	$ 139	$ 0	$ 1
Return on average allocated equity, continuing operations (as a percentage)	(1.66%)	1.62%	(2.25%)
Return on average allocated equity (as a percentage)	(1.56%)	1.71%	(2.16%)
Average full-time equivalent employees | employee	5,521	5,774	6,094
Provision for credit loss, net of tax, related to previously disclosed fraud incident	$ 106