Business Segment Reporting - Schedule of Segment Reporting Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) employee	Sep. 30, 2020 USD ($) employee	Sep. 30, 2021 USD ($) employee	Sep. 30, 2020 USD ($) employee
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 1,025	$ 1,006	$ 3,060	$ 3,020
Noninterest income	797	681	2,285	1,850
Total revenue (TE)	1,822	1,687	5,345	4,870
Provision for credit losses	(107)	160	(422)	1,001
Depreciation and amortization expense	87	89	267	272
Other noninterest expense	1,025	948	2,992	2,709
Income (loss) from continuing operations before income taxes (TE)	817	490	2,508	888
Allocated income taxes and TE adjustments	174	66	523	134
INCOME (LOSS) FROM CONTINUING OPERATIONS	643	424	1,985	754
Income (loss) from discontinued operations, net of taxes	2	4	11	7
NET INCOME (LOSS)	645	428	1,996	761
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	645	428	1,996	761
AVERAGE BALANCES
Loans and leases	100,138	104,919	100,562	103,018
Total assets	181,371	168,811	177,194	159,746
Deposits	146,916	134,944	143,023	124,455
OTHER FINANCIAL DATA
Net loan charge-offs	$ 29	$ 128	$ 165	$ 308
Return on average allocated equity, continuing operations (as a percent)	14.25%	9.51%	14.87%	5.74%
Return on average allocated equity (as a percent)	14.30%	9.60%	14.96%	5.79%
Average full-time equivalent employees | employee	17,009	17,097	17,034	16,758
Operating Segments | Consumer Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 582	$ 598	$ 1,788	$ 1,765
Noninterest income	288	266	798	740
Total revenue (TE)	870	864	2,586	2,505
Provision for credit losses	(38)	(3)	(132)	289
Depreciation and amortization expense	31	19	71	59
Other noninterest expense	560	548	1,706	1,597
Income (loss) from continuing operations before income taxes (TE)	317	300	941	560
Allocated income taxes and TE adjustments	76	71	226	133
INCOME (LOSS) FROM CONTINUING OPERATIONS	241	229	715	427
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	241	229	715	427
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	241	229	715	427
AVERAGE BALANCES
Loans and leases	39,796	38,354	39,883	36,319
Total assets	42,981	43,304	43,094	40,644
Deposits	89,156	82,829	87,549	78,414
OTHER FINANCIAL DATA
Net loan charge-offs	$ 35	$ 23	$ 105	$ 106
Return on average allocated equity, continuing operations (as a percent)	25.81%	26.21%	26.70%	16.66%
Return on average allocated equity (as a percent)	25.81%	26.21%	26.70%	16.66%
Average full-time equivalent employees | employee	7,976	8,323	8,062	8,171
Operating Segments | Commercial Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 414	$ 427	$ 1,244	$ 1,305
Noninterest income	477	384	1,377	1,023
Total revenue (TE)	891	811	2,621	2,328
Provision for credit losses	(69)	150	(267)	697
Depreciation and amortization expense	34	35	101	108
Other noninterest expense	436	412	1,263	1,144
Income (loss) from continuing operations before income taxes (TE)	490	214	1,524	379
Allocated income taxes and TE adjustments	106	41	324	38
INCOME (LOSS) FROM CONTINUING OPERATIONS	384	173	1,200	341
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	384	173	1,200	341
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	384	173	1,200	341
AVERAGE BALANCES
Loans and leases	59,914	66,378	60,358	66,374
Total assets	69,285	74,530	69,607	75,205
Deposits	56,546	51,585	54,660	45,350
OTHER FINANCIAL DATA
Net loan charge-offs	$ (6)	$ 103	$ 81	$ 200
Return on average allocated equity, continuing operations (as a percent)	18.68%	13.35%	18.92%	9.19%
Return on average allocated equity (as a percent)	18.68%	13.35%	18.92%	9.19%
Average full-time equivalent employees | employee	2,371	2,312	2,371	2,288
Other
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 29	$ (19)	$ 28	$ (50)
Noninterest income	32	31	110	87
Total revenue (TE)	61	12	138	37
Provision for credit losses	0	13	(23)	15
Depreciation and amortization expense	22	35	95	105
Other noninterest expense	29	(12)	23	(32)
Income (loss) from continuing operations before income taxes (TE)	10	(24)	43	(51)
Allocated income taxes and TE adjustments	(8)	(46)	(27)	(37)
INCOME (LOSS) FROM CONTINUING OPERATIONS	18	22	70	(14)
Income (loss) from discontinued operations, net of taxes	2	4	11	7
NET INCOME (LOSS)	20	26	81	(7)
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	20	26	81	(7)
AVERAGE BALANCES
Loans and leases	428	187	321	325
Total assets	69,105	50,977	64,493	43,897
Deposits	1,214	530	814	691
OTHER FINANCIAL DATA
Net loan charge-offs	$ 0	$ 2	$ (21)	$ 2
Return on average allocated equity, continuing operations (as a percent)	1.18%	0.96%	1.62%	(0.20%)
Return on average allocated equity (as a percent)	1.31%	1.14%	1.87%	(0.10%)
Average full-time equivalent employees | employee	6,662	6,462	6,601	6,299