Business Segment Reporting - Schedule of Selected Financial Data for Our Business Segments (Details) $ in Millions	3 Months Ended
	Mar. 31, 2023 USD ($) employee	Mar. 31, 2022 USD ($) employee
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 1,106	$ 1,020
Noninterest income	608	676
Total revenue (TE)	1,714	1,696
Provision for credit losses	139	83
Depreciation and amortization expense	61	70
Other noninterest expense	1,115	1,000
Income (loss) from continuing operations before income taxes (TE)	399	543
Allocated income taxes and TE adjustments	88	96
INCOME (LOSS) FROM CONTINUING OPERATIONS	311	447
Income (loss) from discontinued operations, net of taxes	1	1
NET INCOME (LOSS)	312	448
Less: Net income (loss) attributable to noncontrolling interests	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	312	448
AVERAGE BALANCES
Loans and leases	119,837	103,762
Total assets	190,851	182,589
Deposits	143,405	150,163
OTHER FINANCIAL DATA
Net loan charge-offs	$ 45	$ 33
Return on average allocated equity, continuing operations (as a percent)	9.13%	10.80%
Return on average allocated equity (as a percent)	9.16%	10.83%
Average full-time equivalent employees | employee	18,220	17,109
Operating Segments | Consumer Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 614	$ 543
Noninterest income	228	256
Total revenue (TE)	842	799
Provision for credit losses	60	43
Depreciation and amortization expense	21	21
Other noninterest expense	654	642
Income (loss) from continuing operations before income taxes (TE)	107	93
Allocated income taxes and TE adjustments	26	22
INCOME (LOSS) FROM CONTINUING OPERATIONS	81	71
Income (loss) from discontinued operations, net of taxes	0	0
NET INCOME (LOSS)	81	71
Less: Net income (loss) attributable to noncontrolling interests	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	81	71
AVERAGE BALANCES
Loans and leases	43,086	38,654
Total assets	45,911	41,786
Deposits	84,492	91,516
OTHER FINANCIAL DATA
Net loan charge-offs	$ 24	$ 22
Return on average allocated equity, continuing operations (as a percent)	8.98%	8.02%
Return on average allocated equity (as a percent)	8.98%	8.02%
Average full-time equivalent employees | employee	8,046	7,885
Operating Segments | Commercial Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 475	$ 414
Noninterest income	366	394
Total revenue (TE)	841	808
Provision for credit losses	80	41
Depreciation and amortization expense	23	31
Other noninterest expense	405	383
Income (loss) from continuing operations before income taxes (TE)	333	353
Allocated income taxes and TE adjustments	69	69
INCOME (LOSS) FROM CONTINUING OPERATIONS	264	284
Income (loss) from discontinued operations, net of taxes	0	0
NET INCOME (LOSS)	264	284
Less: Net income (loss) attributable to noncontrolling interests	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	264	284
AVERAGE BALANCES
Loans and leases	76,306	64,684
Total assets	85,852	74,816
Deposits	52,185	57,241
OTHER FINANCIAL DATA
Net loan charge-offs	$ 21	$ 11
Return on average allocated equity, continuing operations (as a percent)	10.39%	13.26%
Return on average allocated equity (as a percent)	10.39%	13.26%
Average full-time equivalent employees | employee	2,517	2,416
Other
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 17	$ 63
Noninterest income	14	26
Total revenue (TE)	31	89
Provision for credit losses	(1)	(1)
Depreciation and amortization expense	17	18
Other noninterest expense	56	(25)
Income (loss) from continuing operations before income taxes (TE)	(41)	97
Allocated income taxes and TE adjustments	(7)	5
INCOME (LOSS) FROM CONTINUING OPERATIONS	(34)	92
Income (loss) from discontinued operations, net of taxes	1	1
NET INCOME (LOSS)	(33)	93
Less: Net income (loss) attributable to noncontrolling interests	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	(33)	93
AVERAGE BALANCES
Loans and leases	445	424
Total assets	59,088	65,987
Deposits	6,728	1,406
OTHER FINANCIAL DATA
Net loan charge-offs	$ 0	$ 0
Return on average allocated equity, continuing operations (as a percent)	95.76%	8.29%
Return on average allocated equity (as a percent)	92.94%	8.38%
Average full-time equivalent employees | employee	7,657	6,808