Business Segment Reporting - Schedule of Selected Financial Data for Our Business Segments (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023 USD ($) employee	Jun. 30, 2022 USD ($) employee	Jun. 30, 2023 USD ($) employee	Jun. 30, 2022 USD ($) employee
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 986	$ 1,104	$ 2,092	$ 2,124
Noninterest income	609	688	1,217	1,364
Total revenue (TE)	1,595	1,792	3,309	3,488
Provision for credit losses	167	45	306	128
Depreciation and amortization expense	60	69	121	139
Other noninterest expense	1,016	1,009	2,131	2,009
Income (loss) from continuing operations before income taxes (TE)	352	669	751	1,212
Allocated income taxes and TE adjustments	66	139	154	235
INCOME (LOSS) FROM CONTINUING OPERATIONS	286	530	597	977
Income (loss) from discontinued operations, net of taxes	1	3	2	4
NET INCOME (LOSS)	287	533	599	981
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	287	533	599	981
AVERAGE BALANCES
Loans and leases	120,672	109,138	120,257	106,465
Total assets	196,460	183,527	193,672	183,060
Deposits	142,903	147,471	143,153	148,809
OTHER FINANCIAL DATA
Net loan charge-offs	$ 52	$ 44	$ 97	$ 77
Return on average allocated equity, continuing operations (as a percent)	7.96%	14.76%	8.53%	12.64%
Return on average allocated equity (as a percent)	7.99%	14.85%	8.56%	12.69%
Average full-time equivalent employees | employee	17,754	17,414	17,987	17,262
Operating Segments | Consumer Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 558	$ 604	$ 1,170	$ 1,157
Noninterest income	245	254	473	511
Total revenue (TE)	803	858	1,643	1,668
Provision for credit losses	32	8	92	51
Depreciation and amortization expense	21	22	42	43
Other noninterest expense	642	659	1,284	1,308
Income (loss) from continuing operations before income taxes (TE)	108	169	225	266
Allocated income taxes and TE adjustments	26	41	54	64
INCOME (LOSS) FROM CONTINUING OPERATIONS	82	128	171	202
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	82	128	171	202
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	82	128	171	202
AVERAGE BALANCES
Loans and leases	42,934	40,827	43,009	39,746
Total assets	45,761	43,897	45,847	42,856
Deposits	82,498	91,394	83,561	91,517
OTHER FINANCIAL DATA
Net loan charge-offs	$ 32	$ 23	$ 57	$ 45
Return on average allocated equity, continuing operations (as a percent)	9.04%	13.94%	9.46%	11.20%
Return on average allocated equity (as a percent)	9.04%	13.94%	9.46%	11.20%
Average full-time equivalent employees | employee	7,829	8,074	7,962	8,012
Operating Segments | Commercial Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 459	$ 470	$ 937	$ 893
Noninterest income	346	404	712	798
Total revenue (TE)	805	874	1,649	1,691
Provision for credit losses	134	37	215	78
Depreciation and amortization expense	22	30	46	61
Other noninterest expense	383	381	801	763
Income (loss) from continuing operations before income taxes (TE)	266	426	587	789
Allocated income taxes and TE adjustments	52	86	118	157
INCOME (LOSS) FROM CONTINUING OPERATIONS	214	340	469	632
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	214	340	469	632
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	214	340	469	632
AVERAGE BALANCES
Loans and leases	77,277	67,825	76,794	66,263
Total assets	87,106	78,816	86,483	76,827
Deposits	51,420	54,846	51,818	56,037
OTHER FINANCIAL DATA
Net loan charge-offs	$ 20	$ 21	$ 40	$ 32
Return on average allocated equity, continuing operations (as a percent)	8.17%	15.29%	9.09%	14.48%
Return on average allocated equity (as a percent)	8.17%	15.29%	9.09%	14.48%
Average full-time equivalent employees | employee	2,487	2,408	2,502	2,412
Other
SUMMARY OF OPERATIONS
Net interest income (TE)	$ (31)	$ 30	$ (15)	$ 74
Noninterest income	18	30	32	55
Total revenue (TE)	(13)	60	17	129
Provision for credit losses	1	0	(1)	(1)
Depreciation and amortization expense	17	17	33	35
Other noninterest expense	(9)	(31)	46	(62)
Income (loss) from continuing operations before income taxes (TE)	(22)	74	(61)	157
Allocated income taxes and TE adjustments	(12)	12	(18)	14
INCOME (LOSS) FROM CONTINUING OPERATIONS	(10)	62	(43)	143
Income (loss) from discontinued operations, net of taxes	1	3	2	4
NET INCOME (LOSS)	(9)	65	(41)	147
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	(9)	65	(41)	147
AVERAGE BALANCES
Loans and leases	461	486	454	456
Total assets	63,593	60,814	61,342	63,377
Deposits	8,985	1,231	7,774	1,255
OTHER FINANCIAL DATA
Net loan charge-offs	$ 0	$ 0	$ 0	$ 0
Return on average allocated equity, continuing operations (as a percent)	(14.75%)	13.84%	(133.40%)	9.18%
Return on average allocated equity (as a percent)	(13.27%)	14.51%	(127.20%)	9.43%
Average full-time equivalent employees | employee	7,438	6,932	7,523	6,838