Business Segment Reporting - Schedule of Selected Financial Data for Our Business Segments (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024 USD ($) employee	Jun. 30, 2023 USD ($) employee	Jun. 30, 2024 USD ($) employee	Jun. 30, 2023 USD ($) employee
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 899	$ 986	$ 1,785	$ 2,092
Noninterest income	627	609	1,274	1,217
Total revenue (TE)	1,526	1,595	3,059	3,309
Provision for credit losses	100	167	201	306
Depreciation and amortization expense	54	60	108	121
Other noninterest expense	1,025	1,016	2,114	2,131
Income (loss) from continuing operations before income taxes (TE)	347	352	636	751
Allocated income taxes and TE adjustments	74	66	144	154
INCOME (LOSS) FROM CONTINUING OPERATIONS	273	286	492	597
Income (loss) from discontinued operations, net of taxes	1	1	1	2
NET INCOME (LOSS)	274	287	493	599
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	274	287	493	599
AVERAGE BALANCES
Loans and leases	108,961	120,672	109,997	120,257
Total assets	186,583	196,460	186,225	193,672
Deposits	144,180	142,903	143,530	143,153
OTHER FINANCIAL DATA
Net loan charge-offs	$ 91	$ 52	$ 172	$ 97
Return on average allocated equity, continuing operations (as a percent)	7.57%	7.96%	6.78%	8.53%
Return on average allocated equity	7.59%	7.99%	6.79%	8.56%
Average full-time equivalent employees | employee	16,646	17,754	16,699	17,987
Operating Segments | Consumer Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 535	$ 544	$ 1,067	$ 1,143
Noninterest income	234	243	458	468
Total revenue (TE)	769	787	1,525	1,611
Provision for credit losses	33	32	31	92
Depreciation and amortization expense	18	21	41	42
Other noninterest expense	630	641	1,310	1,282
Income (loss) from continuing operations before income taxes (TE)	88	93	143	195
Allocated income taxes and TE adjustments	21	22	34	46
INCOME (LOSS) FROM CONTINUING OPERATIONS	67	71	109	149
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	67	71	109	149
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	67	71	109	149
AVERAGE BALANCES
Loans and leases	39,174	42,297	39,547	42,377
Total assets	42,008	45,116	42,359	45,206
Deposits	85,397	81,406	84,736	82,460
OTHER FINANCIAL DATA
Net loan charge-offs	$ 45	$ 32	$ 89	$ 57
Return on average allocated equity, continuing operations (as a percent)	7.93%	8.00%	6.34%	8.41%
Return on average allocated equity	7.93%	8.00%	6.34%	8.41%
Average full-time equivalent employees | employee	7,337	7,811	7,347	7,944
Operating Segments | Commercial Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 411	$ 475	$ 808	$ 968
Noninterest income	358	348	759	717
Total revenue (TE)	769	823	1,567	1,685
Provision for credit losses	87	134	189	215
Depreciation and amortization expense	19	22	42	46
Other noninterest expense	412	384	831	803
Income (loss) from continuing operations before income taxes (TE)	251	283	505	621
Allocated income taxes and TE adjustments	44	56	93	126
INCOME (LOSS) FROM CONTINUING OPERATIONS	207	227	412	495
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	207	227	412	495
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	207	227	412	495
AVERAGE BALANCES
Loans and leases	69,248	77,922	69,940	77,435
Total assets	78,328	87,759	79,164	87,132
Deposits	57,360	52,512	56,846	52,918
OTHER FINANCIAL DATA
Net loan charge-offs	$ 64	$ 20	$ 102	$ 40
Return on average allocated equity, continuing operations (as a percent)	8.31%	8.61%	8.27%	9.52%
Return on average allocated equity	8.31%	8.61%	8.27%	9.52%
Average full-time equivalent employees | employee	2,320	2,505	2,327	2,520
Other
SUMMARY OF OPERATIONS
Net interest income (TE)	$ (47)	$ (33)	$ (90)	$ (19)
Noninterest income	35	18	57	32
Total revenue (TE)	(12)	(15)	(33)	13
Provision for credit losses	(20)	1	(19)	(1)
Depreciation and amortization expense	17	17	25	33
Other noninterest expense	(17)	(9)	(27)	46
Income (loss) from continuing operations before income taxes (TE)	8	(24)	(12)	(65)
Allocated income taxes and TE adjustments	9	(12)	17	(18)
INCOME (LOSS) FROM CONTINUING OPERATIONS	(1)	(12)	(29)	(47)
Income (loss) from discontinued operations, net of taxes	1	1	1	2
NET INCOME (LOSS)	0	(11)	(28)	(45)
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	0	(11)	(28)	(45)
AVERAGE BALANCES
Loans and leases	539	453	510	445
Total assets	66,247	63,585	64,702	61,334
Deposits	1,423	8,985	1,948	7,775
OTHER FINANCIAL DATA
Net loan charge-offs	$ (18)	$ 0	$ (19)	$ 0
Return on average allocated equity, continuing operations (as a percent)	(0.37%)	(17.70%)	(5.17%)	(145.81%)
Return on average allocated equity	0.00%	(16.22%)	(4.99%)	(139.61%)
Average full-time equivalent employees | employee	6,989	7,438	7,025	7,523