Business Segment Reporting - Schedule of Segment Reporting Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) employee	Jun. 30, 2019 USD ($) employee	Jun. 30, 2020 USD ($) employee	Jun. 30, 2019 USD ($) employee
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 1,025	$ 989	$ 2,014	$ 1,974
Noninterest income	692	622	1,169	1,158
Total revenue (TE)	1,717	1,611	3,183	3,132
Provision for credit losses	482	74	841	136
Depreciation and amortization expense	91	97	182	185
Other noninterest expense	922	922	1,762	1,797
Income (loss) from continuing operations before income taxes (TE)	222	518	398	1,014
Allocated income taxes and TE adjustments	37	95	68	185
INCOME (LOSS) FROM CONTINUING OPERATIONS	185	423	330	829
Income (loss) from discontinued operations, net of taxes	2	2	3	3
NET INCOME (LOSS)	187	425	333	832
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	187	425	333	832
AVERAGE BALANCES
Loans and leases	107,941	90,785	102,058	90,220
Total assets	164,099	142,635	155,164	141,385
Deposits	127,977	109,601	119,153	108,594
OTHER FINANCIAL DATA
Net loan charge-offs	$ 97	$ 65	$ 181	$ 129
Return on average allocated equity, continuing operations	4.21%	10.26%	3.80%	10.37%
Return on average allocated equity	4.25%	10.31%	3.84%	10.41%
Average full-time equivalent employees | employee	16,646	17,206	16,587	17,379
Operating Segments | Consumer Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 594	$ 594	$ 1,184	$ 1,185
Noninterest income	247	231	478	445
Total revenue (TE)	841	825	1,662	1,630
Provision for credit losses	167	40	307	85
Depreciation and amortization expense	20	25	41	48
Other noninterest expense	535	527	1,058	1,044
Income (loss) from continuing operations before income taxes (TE)	119	233	256	453
Allocated income taxes and TE adjustments	28	56	61	108
INCOME (LOSS) FROM CONTINUING OPERATIONS	91	177	195	345
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	91	177	195	345
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	91	177	195	345
AVERAGE BALANCES
Loans and leases	39,197	31,881	37,197	31,603
Total assets	44,106	35,469	41,283	35,103
Deposits	79,502	72,303	76,411	71,798
OTHER FINANCIAL DATA
Net loan charge-offs	$ 39	$ 40	$ 83	$ 74
Return on average allocated equity, continuing operations	10.38%	21.75%	11.21%	21.51%
Return on average allocated equity	10.38%	21.75%	11.21%	21.51%
Average full-time equivalent employees | employee	8,863	9,440	8,885	9,531
Operating Segments | Commercial Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 442	$ 405	$ 852	$ 807
Noninterest income	405	355	624	654
Total revenue (TE)	847	760	1,476	1,461
Provision for credit losses	314	33	529	49
Depreciation and amortization expense	37	35	73	65
Other noninterest expense	366	354	681	697
Income (loss) from continuing operations before income taxes (TE)	130	338	193	650
Allocated income taxes and TE adjustments	10	61	3	123
INCOME (LOSS) FROM CONTINUING OPERATIONS	120	277	190	527
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	120	277	190	527
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	120	277	190	527
AVERAGE BALANCES
Loans and leases	68,038	57,918	64,060	57,594
Total assets	76,974	65,901	73,178	65,390
Deposits	46,099	35,960	41,078	35,193
OTHER FINANCIAL DATA
Net loan charge-offs	$ 57	$ 23	$ 97	$ 53
Return on average allocated equity, continuing operations	10.00%	24.09%	8.03%	23.36%
Return on average allocated equity	10.00%	24.09%	8.03%	23.36%
Average full-time equivalent employees | employee	2,096	2,260	2,082	2,314
Other
SUMMARY OF OPERATIONS
Net interest income (TE)	$ (11)	$ (10)	$ (22)	$ (18)
Noninterest income	40	36	67	59
Total revenue (TE)	29	26	45	41
Provision for credit losses	1	1	5	2
Depreciation and amortization expense	34	37	68	72
Other noninterest expense	21	41	23	56
Income (loss) from continuing operations before income taxes (TE)	(27)	(53)	(51)	(89)
Allocated income taxes and TE adjustments	(1)	(22)	4	(46)
INCOME (LOSS) FROM CONTINUING OPERATIONS	(26)	(31)	(55)	(43)
Income (loss) from discontinued operations, net of taxes	2	2	3	3
NET INCOME (LOSS)	(24)	(29)	(52)	(40)
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	(24)	(29)	(52)	(40)
AVERAGE BALANCES
Loans and leases	706	986	801	1,023
Total assets	43,019	41,265	40,703	40,892
Deposits	2,376	1,338	1,664	1,603
OTHER FINANCIAL DATA
Net loan charge-offs	$ 1	$ 2	$ 1	$ 2
Return on average allocated equity, continuing operations	(1.12%)	(1.44%)	(1.20%)	(1.04%)
Return on average allocated equity	(1.03%)	(1.34%)	(1.14%)	(0.97%)
Average full-time equivalent employees | employee	5,687	5,506	5,620	5,534