Business Segment Reporting - Schedule of Segment Reporting Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2022 USD ($) employee	Mar. 31, 2021 USD ($) employee
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 1,020	$ 1,012
Noninterest income	676	738
Total revenue (TE)	1,696	1,750
Provision for credit losses	83	(93)
Depreciation and amortization expense	70	75
Other noninterest expense	1,000	996
Income (loss) from continuing operations before income taxes (TE)	543	772
Allocated income taxes and TE adjustments	96	154
INCOME (LOSS) FROM CONTINUING OPERATIONS	447	618
Income (loss) from discontinued operations, net of taxes	1	4
NET INCOME (LOSS)	448	622
Less: Net income (loss) attributable to noncontrolling interests	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	448	622
AVERAGE BALANCES
Loans and leases	103,762	100,728
Total assets	182,589	172,233
Deposits	150,163	137,740
OTHER FINANCIAL DATA
Net loan charge-offs	$ 33	$ 114
Return on average allocated equity, continuing operations (as a percent)	10.80%	14.11%
Return on average allocated equity (as a percent)	10.83%	14.20%
Average full-time equivalent employees | employee	17,110	17,086
Operating Segments | Consumer Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 543	$ 607
Noninterest income	256	257
Total revenue (TE)	799	864
Provision for credit losses	43	(23)
Depreciation and amortization expense	21	18
Other noninterest expense	642	583
Income (loss) from continuing operations before income taxes (TE)	93	286
Allocated income taxes and TE adjustments	23	69
INCOME (LOSS) FROM CONTINUING OPERATIONS	70	217
Income (loss) from discontinued operations, net of taxes	0	0
NET INCOME (LOSS)	70	217
Less: Net income (loss) attributable to noncontrolling interests	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	70	217
AVERAGE BALANCES
Loans and leases	38,637	39,249
Total assets	41,814	42,476
Deposits	91,468	85,033
OTHER FINANCIAL DATA
Net loan charge-offs	$ 22	$ 36
Return on average allocated equity, continuing operations (as a percent)	7.91%	25.74%
Return on average allocated equity (as a percent)	7.91%	25.74%
Average full-time equivalent employees | employee	7,899	8,284
Operating Segments | Commercial Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 415	$ 411
Noninterest income	395	447
Total revenue (TE)	810	858
Provision for credit losses	41	(67)
Depreciation and amortization expense	31	34
Other noninterest expense	386	409
Income (loss) from continuing operations before income taxes (TE)	352	482
Allocated income taxes and TE adjustments	69	99
INCOME (LOSS) FROM CONTINUING OPERATIONS	283	383
Income (loss) from discontinued operations, net of taxes	0	0
NET INCOME (LOSS)	283	383
Less: Net income (loss) attributable to noncontrolling interests	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	283	383
AVERAGE BALANCES
Loans and leases	64,701	61,221
Total assets	74,860	70,448
Deposits	57,289	51,894
OTHER FINANCIAL DATA
Net loan charge-offs	$ 11	$ 78
Return on average allocated equity, continuing operations (as a percent)	13.21%	17.41%
Return on average allocated equity (as a percent)	13.21%	17.41%
Average full-time equivalent employees | employee	2,402	2,258
Other
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 62	$ (6)
Noninterest income	25	34
Total revenue (TE)	87	28
Provision for credit losses	(1)	(3)
Depreciation and amortization expense	18	23
Other noninterest expense	(28)	4
Income (loss) from continuing operations before income taxes (TE)	98	4
Allocated income taxes and TE adjustments	4	(14)
INCOME (LOSS) FROM CONTINUING OPERATIONS	94	18
Income (loss) from discontinued operations, net of taxes	1	4
NET INCOME (LOSS)	95	22
Less: Net income (loss) attributable to noncontrolling interests	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	95	22
AVERAGE BALANCES
Loans and leases	424	258
Total assets	65,915	59,309
Deposits	1,406	813
OTHER FINANCIAL DATA
Net loan charge-offs	$ 0	$ 0
Return on average allocated equity, continuing operations (as a percent)	8.47%	1.35%
Return on average allocated equity (as a percent)	8.56%	1.64%
Average full-time equivalent employees | employee	6,809	6,544