Business Segment Reporting - Schedule of Segment Reporting Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022 USD ($) employee	Sep. 30, 2021 USD ($) employee	Sep. 30, 2022 USD ($) employee	Sep. 30, 2021 USD ($) employee
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 1,203	$ 1,025	$ 3,327	$ 3,060
Noninterest income	683	797	2,047	2,285
Total revenue (TE)	1,886	1,822	5,374	5,345
Provision for credit losses	109	(107)	237	(422)
Depreciation and amortization expense	68	75	207	226
Other noninterest expense	1,038	1,037	3,047	3,033
Income (loss) from continuing operations before income taxes (TE)	671	817	1,883	2,508
Allocated income taxes and TE adjustments	131	174	366	523
INCOME (LOSS) FROM CONTINUING OPERATIONS	540	643	1,517	1,985
Income (loss) from discontinued operations, net of taxes	2	2	6	11
NET INCOME (LOSS)	542	645	1,523	1,996
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	542	645	1,523	1,996
AVERAGE BALANCES
Loans and leases	114,418	100,138	109,144	100,562
Total assets	188,192	181,371	184,789	177,194
Deposits	144,229	146,916	147,266	143,023
OTHER FINANCIAL DATA
Net loan charge-offs	$ 43	$ 29	$ 120	$ 165
Return on average allocated equity, continuing operations (as a percent)	14.66%	14.25%	13.29%	14.87%
Return on average allocated equity (as a percent)	14.71%	14.30%	13.35%	14.96%
Average full-time equivalent employees | employee	17,907	17,009	17,477	17,034
Operating Segments | Consumer Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 632	$ 582	$ 1,745	$ 1,789
Noninterest income	259	288	769	798
Total revenue (TE)	891	870	2,514	2,587
Provision for credit losses	37	(38)	88	(132)
Depreciation and amortization expense	22	23	65	61
Other noninterest expense	645	568	1,941	1,717
Income (loss) from continuing operations before income taxes (TE)	187	317	420	941
Allocated income taxes and TE adjustments	45	76	101	226
INCOME (LOSS) FROM CONTINUING OPERATIONS	142	241	319	715
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	142	241	319	715
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	142	241	319	715
AVERAGE BALANCES
Loans and leases	42,568	39,854	40,697	39,954
Total assets	45,638	43,039	43,782	43,165
Deposits	90,044	89,278	90,939	87,667
OTHER FINANCIAL DATA
Net loan charge-offs	$ 17	$ 35	$ 62	$ 105
Return on average allocated equity, continuing operations (as a percent)	16.20%	25.81%	11.90%	26.70%
Return on average allocated equity (as a percent)	16.20%	25.81%	11.90%	26.70%
Average full-time equivalent employees | employee	8,114	7,976	8,013	8,062
Operating Segments | Commercial Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 495	$ 407	$ 1,347	$ 1,234
Noninterest income	394	477	1,192	1,379
Total revenue (TE)	889	884	2,539	2,613
Provision for credit losses	74	(69)	152	(267)
Depreciation and amortization expense	28	34	89	101
Other noninterest expense	422	436	1,184	1,263
Income (loss) from continuing operations before income taxes (TE)	365	483	1,114	1,516
Allocated income taxes and TE adjustments	70	104	218	322
INCOME (LOSS) FROM CONTINUING OPERATIONS	295	379	896	1,194
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	295	379	896	1,194
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	295	379	896	1,194
AVERAGE BALANCES
Loans and leases	71,464	59,856	68,016	60,287
Total assets	81,898	69,227	78,536	69,536
Deposits	52,272	56,401	54,768	54,309
OTHER FINANCIAL DATA
Net loan charge-offs	$ 27	$ (6)	$ 59	$ 81
Return on average allocated equity, continuing operations (as a percent)	12.63%	18.02%	13.37%	18.65%
Return on average allocated equity (as a percent)	12.63%	18.02%	13.37%	18.65%
Average full-time equivalent employees | employee	2,523	2,371	2,441	2,371
Other
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 76	$ 36	$ 235	$ 37
Noninterest income	30	32	86	108
Total revenue (TE)	106	68	321	145
Provision for credit losses	(2)	0	(3)	(23)
Depreciation and amortization expense	18	18	53	64
Other noninterest expense	(29)	33	(78)	53
Income (loss) from continuing operations before income taxes (TE)	119	17	349	51
Allocated income taxes and TE adjustments	16	(6)	47	(25)
INCOME (LOSS) FROM CONTINUING OPERATIONS	103	23	302	76
Income (loss) from discontinued operations, net of taxes	2	2	6	11
NET INCOME (LOSS)	105	25	308	87
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	105	25	308	87
AVERAGE BALANCES
Loans and leases	386	428	431	321
Total assets	60,656	69,105	62,471	64,493
Deposits	1,913	1,237	1,559	1,047
OTHER FINANCIAL DATA
Net loan charge-offs	$ (1)	$ 0	$ (1)	$ (21)
Return on average allocated equity, continuing operations (as a percent)	21.83%	1.56%	14.88%	1.78%
Return on average allocated equity (as a percent)	22.25%	1.70%	15.17%	2.04%
Average full-time equivalent employees | employee	7,270	6,662	7,023	6,601