Business Segment Reporting - Financial Information of Business Groups (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) employee	Dec. 31, 2020 USD ($) employee	Dec. 31, 2019 USD ($) employee
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 4,098	$ 4,063	$ 3,941
Noninterest income	3,194	2,652	2,459
Total revenue (TE)	7,292	6,715	6,400
Provision for credit losses	(418)	1,021	445
Depreciation and amortization expense	298	303	313
Other noninterest expense	4,131	3,806	3,588
Income (loss) from continuing operations before income taxes (TE)	3,281	1,585	2,054
Allocated income taxes (benefit) and TE adjustments	669	256	346
INCOME (LOSS) FROM CONTINUING OPERATIONS	2,612	1,329	1,708
Income (loss) from discontinued operations, net of taxes	13	14	9
NET INCOME (LOSS)	2,625	1,343	1,717
Less: Net income (loss) attributable to noncontrolling interests	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	2,625	1,343	1,717
AVERAGE BALANCES
Loans and leases	100,269	102,689	91,511
Total assets	178,919	162,055	143,179
Deposits	145,035	127,286	110,030
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	114	161	245
Net loan charge-offs	$ 185	$ 444	$ 432
Return on average allocated equity, continuing operations (as a percentage)	14.79%	7.54%	10.27%
Return on average allocated equity (as a percentage)	14.86%	7.62%	10.32%
Average full-time equivalent employees | employee	16,974	16,826	17,045
Provision for credit loss, net of tax, related to previously disclosed fraud incident			$ (139)
Operating Segments | Consumer Bank
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 2,357	$ 2,403	2,335
Noninterest income	1,068	999	917
Total revenue (TE)	3,425	3,402	3,252
Provision for credit losses	(118)	284	186
Depreciation and amortization expense	84	77	91
Other noninterest expense	2,306	2,185	2,062
Income (loss) from continuing operations before income taxes (TE)	1,153	856	913
Allocated income taxes (benefit) and TE adjustments	277	203	216
INCOME (LOSS) FROM CONTINUING OPERATIONS	876	653	697
Income (loss) from discontinued operations, net of taxes	0	0	0
NET INCOME (LOSS)	876	653	697
Less: Net income (loss) attributable to noncontrolling interests	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	876	653	697
AVERAGE BALANCES
Loans and leases	39,356	37,842	30,571
Total assets	42,572	41,152	34,111
Deposits	88,235	79,528	72,384
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	39	40	150
Net loan charge-offs	$ 126	$ 135	$ 157
Return on average allocated equity, continuing operations (as a percentage)	24.54%	18.97%	21.59%
Return on average allocated equity (as a percentage)	24.54%	18.97%	21.59%
Average full-time equivalent employees | employee	8,000	8,186	8,540
Operating Segments | Commercial Bank
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 1,655	$ 1,725	$ 1,651
Noninterest income	1,989	1,524	1,395
Total revenue (TE)	3,644	3,249	3,046
Provision for credit losses	(279)	741	120
Depreciation and amortization expense	134	144	135
Other noninterest expense	1,731	1,606	1,424
Income (loss) from continuing operations before income taxes (TE)	2,058	758	1,367
Allocated income taxes (benefit) and TE adjustments	413	107	223
INCOME (LOSS) FROM CONTINUING OPERATIONS	1,645	651	1,144
Income (loss) from discontinued operations, net of taxes	0	0	0
NET INCOME (LOSS)	1,645	651	1,144
Less: Net income (loss) attributable to noncontrolling interests	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	1,645	651	1,144
AVERAGE BALANCES
Loans and leases	60,552	64,543	60,417
Total assets	70,117	74,225	68,562
Deposits	55,715	47,145	36,372
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	12	1	(8)
Net loan charge-offs	$ 81	$ 308	$ 128
Return on average allocated equity, continuing operations (as a percentage)	19.52%	12.99%	24.54%
Return on average allocated equity (as a percentage)	19.52%	12.99%	24.54%
Average full-time equivalent employees | employee	2,370	2,291	2,438
Other
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 86	$ (65)	$ (45)
Noninterest income	137	129	147
Total revenue (TE)	223	64	102
Provision for credit losses	(21)	(4)	139
Depreciation and amortization expense	80	82	87
Other noninterest expense	94	15	102
Income (loss) from continuing operations before income taxes (TE)	70	(29)	(226)
Allocated income taxes (benefit) and TE adjustments	(21)	(54)	(93)
INCOME (LOSS) FROM CONTINUING OPERATIONS	91	25	(133)
Income (loss) from discontinued operations, net of taxes	13	14	9
NET INCOME (LOSS)	104	39	(124)
Less: Net income (loss) attributable to noncontrolling interests	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	104	39	(124)
AVERAGE BALANCES
Loans and leases	361	304	523
Total assets	66,230	46,678	40,506
Deposits	1,085	613	1,274
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	63	120	103
Net loan charge-offs	$ (22)	$ 1	$ 147
Return on average allocated equity, continuing operations (as a percentage)	1.61%	0.27%	(1.52%)
Return on average allocated equity (as a percentage)	1.84%	0.42%	(1.42%)
Average full-time equivalent employees | employee	6,604	6,349	6,067
Provision for credit loss, net of tax, related to previously disclosed fraud incident	$ 106