Business Segment Reporting - Schedule of Segment Reporting Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022 USD ($) employee	Jun. 30, 2021 USD ($) employee	Jun. 30, 2022 USD ($) employee	Jun. 30, 2021 USD ($) employee
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 1,104	$ 1,023	$ 2,124	$ 2,035
Noninterest income	688	750	1,364	1,488
Total revenue (TE)	1,792	1,773	3,488	3,523
Provision for credit losses	45	(222)	128	(315)
Depreciation and amortization expense	69	76	139	151
Other noninterest expense	1,009	1,000	2,009	1,996
Income (loss) from continuing operations before income taxes (TE)	669	919	1,212	1,691
Allocated income taxes and TE adjustments	139	195	235	349
INCOME (LOSS) FROM CONTINUING OPERATIONS	530	724	977	1,342
Income (loss) from discontinued operations, net of taxes	3	5	4	9
NET INCOME (LOSS)	533	729	981	1,351
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	533	729	981	1,351
AVERAGE BALANCES
Loans and leases	109,138	100,814	106,465	100,777
Total assets	183,527	177,870	183,060	175,071
Deposits	147,471	144,322	148,809	141,044
OTHER FINANCIAL DATA
Net loan charge-offs	$ 44	$ 22	$ 77	$ 136
Return on average allocated equity, continuing operations (as a percent)	14.76%	16.26%	12.64%	15.19%
Return on average allocated equity (as a percent)	14.85%	16.37%	12.69%	15.29%
Average full-time equivalent employees | employee	17,414	17,003	17,262	17,046
Operating Segments | Consumer Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 570	$ 599	$ 1,113	$ 1,206
Noninterest income	254	253	510	511
Total revenue (TE)	824	852	1,623	1,717
Provision for credit losses	8	(70)	51	(94)
Depreciation and amortization expense	22	20	43	37
Other noninterest expense	654	564	1,296	1,150
Income (loss) from continuing operations before income taxes (TE)	140	338	233	624
Allocated income taxes and TE adjustments	33	81	56	150
INCOME (LOSS) FROM CONTINUING OPERATIONS	107	257	177	474
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	107	257	177	474
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	107	257	177	474
AVERAGE BALANCES
Loans and leases	40,818	40,598	39,734	39,927
Total assets	43,868	43,818	42,847	43,151
Deposits	91,256	88,412	91,361	86,732
OTHER FINANCIAL DATA
Net loan charge-offs	$ 23	$ 34	$ 45	$ 70
Return on average allocated equity, continuing operations (as a percent)	11.66%	28.53%	9.81%	27.18%
Return on average allocated equity (as a percent)	11.66%	28.53%	9.81%	27.18%
Average full-time equivalent employees | employee	8,024	7,929	7,962	8,106
Operating Segments | Commercial Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 440	$ 417	$ 855	$ 828
Noninterest income	404	454	799	901
Total revenue (TE)	844	871	1,654	1,729
Provision for credit losses	37	(131)	78	(198)
Depreciation and amortization expense	30	33	61	67
Other noninterest expense	384	418	770	827
Income (loss) from continuing operations before income taxes (TE)	393	551	745	1,033
Allocated income taxes and TE adjustments	78	119	147	218
INCOME (LOSS) FROM CONTINUING OPERATIONS	315	432	598	815
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	315	432	598	815
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	315	432	598	815
AVERAGE BALANCES
Loans and leases	67,834	59,953	66,276	60,584
Total assets	78,824	69,101	76,853	69,771
Deposits	54,864	54,814	56,070	53,362
OTHER FINANCIAL DATA
Net loan charge-offs	$ 21	$ 9	$ 32	$ 87
Return on average allocated equity, continuing operations (as a percent)	14.16%	20.69%	13.70%	19.01%
Return on average allocated equity (as a percent)	14.16%	20.69%	13.70%	19.01%
Average full-time equivalent employees | employee	2,396	2,483	2,399	2,371
Other
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 94	$ 7	$ 156	$ 1
Noninterest income	30	43	55	76
Total revenue (TE)	124	50	211	77
Provision for credit losses	0	(21)	(1)	(23)
Depreciation and amortization expense	17	23	35	47
Other noninterest expense	(29)	18	(57)	19
Income (loss) from continuing operations before income taxes (TE)	136	30	234	34
Allocated income taxes and TE adjustments	28	(5)	32	(19)
INCOME (LOSS) FROM CONTINUING OPERATIONS	108	35	202	53
Income (loss) from discontinued operations, net of taxes	3	5	4	9
NET INCOME (LOSS)	111	40	206	62
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	111	40	206	62
AVERAGE BALANCES
Loans and leases	486	263	455	266
Total assets	60,835	64,951	63,360	62,149
Deposits	1,351	1,096	1,378	950
OTHER FINANCIAL DATA
Net loan charge-offs	$ 0	$ (21)	$ 0	$ (21)
Return on average allocated equity, continuing operations (as a percent)	24.11%	2.39%	12.96%	1.89%
Return on average allocated equity (as a percent)	24.78%	2.73%	13.22%	2.21%
Average full-time equivalent employees | employee	6,994	6,591	6,901	6,569