Business Segment Reporting - Schedule of Segment Reporting Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) employee	Jun. 30, 2020 USD ($) employee	Jun. 30, 2021 USD ($) employee	Jun. 30, 2020 USD ($) employee
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 1,023	$ 1,025	$ 2,035	$ 2,014
Noninterest income	750	692	1,488	1,169
Total revenue (TE)	1,773	1,717	3,523	3,183
Provision for credit losses	(222)	482	(315)	841
Depreciation and amortization expense	90	91	180	182
Other noninterest expense	986	922	1,967	1,762
Income (loss) from continuing operations before income taxes (TE)	919	222	1,691	398
Allocated income taxes and TE adjustments	195	37	349	68
INCOME (LOSS) FROM CONTINUING OPERATIONS	724	185	1,342	330
Income (loss) from discontinued operations, net of taxes	5	2	9	3
NET INCOME (LOSS)	729	187	1,351	333
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	729	187	1,351	333
AVERAGE BALANCES
Loans and leases	100,814	107,941	100,777	102,058
Total assets	177,870	164,099	175,071	155,164
Deposits	144,322	127,977	141,044	119,153
OTHER FINANCIAL DATA
Net loan charge-offs	$ 22	$ 96	$ 136	$ 180
Return on average allocated equity, continuing operations (as a percent)	16.81%	4.21%	15.45%	3.80%
Return on average allocated equity (as a percent)	16.93%	4.25%	15.55%	3.84%
Average full-time equivalent employees | employee	17,157	16,646	17,122	16,587
Operating Segments | Consumer Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 600	$ 589	$ 1,210	$ 1,172
Noninterest income	254	246	511	474
Total revenue (TE)	854	835	1,721	1,646
Provision for credit losses	(70)	155	(94)	292
Depreciation and amortization expense	22	20	40	41
Other noninterest expense	562	532	1,146	1,048
Income (loss) from continuing operations before income taxes (TE)	340	128	629	265
Allocated income taxes and TE adjustments	81	30	150	61
INCOME (LOSS) FROM CONTINUING OPERATIONS	259	98	479	204
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	259	98	479	204
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	259	98	479	204
AVERAGE BALANCES
Loans and leases	40,598	37,300	39,927	35,242
Total assets	43,991	42,194	43,237	39,310
Deposits	88,412	79,235	86,732	76,184
OTHER FINANCIAL DATA
Net loan charge-offs	$ 34	$ 39	$ 70	$ 83
Return on average allocated equity, continuing operations (as a percent)	28.74%	11.50%	27.46%	12.07%
Return on average allocated equity (as a percent)	28.74%	11.50%	27.46%	12.07%
Average full-time equivalent employees | employee	7,929	8,046	8,106	8,095
Operating Segments | Commercial Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 419	$ 458	$ 832	$ 881
Noninterest income	455	421	901	639
Total revenue (TE)	874	879	1,733	1,520
Provision for credit losses	(131)	326	(198)	547
Depreciation and amortization expense	33	37	67	73
Other noninterest expense	418	404	827	732
Income (loss) from continuing operations before income taxes (TE)	554	112	1,037	168
Allocated income taxes and TE adjustments	120	6	218	(4)
INCOME (LOSS) FROM CONTINUING OPERATIONS	434	106	819	172
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	434	106	819	172
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	434	106	819	172
AVERAGE BALANCES
Loans and leases	59,953	70,336	60,584	66,430
Total assets	69,101	79,267	69,771	75,547
Deposits	54,814	47,954	53,362	42,199
OTHER FINANCIAL DATA
Net loan charge-offs	$ 9	$ 57	$ 87	$ 97
Return on average allocated equity, continuing operations (as a percent)	20.79%	8.66%	19.10%	7.13%
Return on average allocated equity (as a percent)	20.79%	8.66%	19.10%	7.13%
Average full-time equivalent employees | employee	2,483	2,293	2,371	2,277
Other
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 4	$ (22)	$ (7)	$ (39)
Noninterest income	41	25	76	56
Total revenue (TE)	45	3	69	17
Provision for credit losses	(21)	1	(23)	2
Depreciation and amortization expense	35	34	73	68
Other noninterest expense	6	(14)	(6)	(18)
Income (loss) from continuing operations before income taxes (TE)	25	(18)	25	(35)
Allocated income taxes and TE adjustments	(6)	1	(19)	11
INCOME (LOSS) FROM CONTINUING OPERATIONS	31	(19)	44	(46)
Income (loss) from discontinued operations, net of taxes	5	2	9	3
NET INCOME (LOSS)	36	(17)	53	(43)
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	36	(17)	53	(43)
AVERAGE BALANCES
Loans and leases	263	305	266	386
Total assets	64,778	42,638	62,063	40,307
Deposits	1,096	788	950	770
OTHER FINANCIAL DATA
Net loan charge-offs	$ (21)	$ 0	$ (21)	$ 0
Return on average allocated equity, continuing operations (as a percent)	2.35%	(0.82%)	1.66%	(1.01%)
Return on average allocated equity (as a percent)	2.73%	(0.73%)	2.00%	(0.94%)
Average full-time equivalent employees | employee	6,745	6,307	6,645	6,215