Business Segment Reporting - Schedule of Segment Reporting Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) employee	Sep. 30, 2019 USD ($) employee	Sep. 30, 2020 USD ($) employee	Sep. 30, 2019 USD ($) employee
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 1,006	$ 980	$ 3,020	$ 2,954
Noninterest income	681	650	1,850	1,808
Total revenue (TE)	1,687	1,630	4,870	4,762
Provision for credit losses	160	200	1,001	336
Depreciation and amortization expense	89	98	272	283
Other noninterest expense	948	841	2,709	2,638
Income (loss) from continuing operations before income taxes (TE)	490	491	888	1,505
Allocated income taxes and TE adjustments	66	78	134	263
INCOME (LOSS) FROM CONTINUING OPERATIONS	424	413	754	1,242
Income (loss) from discontinued operations, net of taxes	4	3	7	6
NET INCOME (LOSS)	428	416	761	1,248
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	428	416	761	1,248
AVERAGE BALANCES
Loans and leases	104,919	91,956	103,018	90,805
Total assets	168,811	144,158	159,746	142,319
Deposits	134,944	110,279	124,455	109,162
OTHER FINANCIAL DATA
Net loan charge-offs	$ 127	$ 197	$ 308	$ 326
Return on average allocated equity, continuing operations (as a percent)	9.51%	9.57%	5.74%	10.09%
Return on average allocated equity (as a percent)	9.60%	9.64%	5.79%	10.14%
Average full-time equivalent employees | employee	17,097	16,898	16,758	17,217
Operating Segments | Consumer Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 604	$ 595	$ 1,788	$ 1,780
Noninterest income	267	238	745	683
Total revenue (TE)	871	833	2,533	2,463
Provision for credit losses	(16)	48	291	133
Depreciation and amortization expense	19	25	59	71
Other noninterest expense	552	504	1,610	1,547
Income (loss) from continuing operations before income taxes (TE)	316	256	573	712
Allocated income taxes and TE adjustments	75	60	136	169
INCOME (LOSS) FROM CONTINUING OPERATIONS	241	196	437	543
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	241	196	437	543
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	241	196	437	543
AVERAGE BALANCES
Loans and leases	41,471	32,760	38,632	31,993
Total assets	44,888	36,397	42,481	35,524
Deposits	83,175	72,995	78,682	72,201
OTHER FINANCIAL DATA
Net loan charge-offs	$ 23	$ 40	$ 106	$ 114
Return on average allocated equity, continuing operations (as a percent)	27.03%	23.22%	16.69%	22.24%
Return on average allocated equity (as a percent)	27.03%	23.22%	16.69%	22.24%
Average full-time equivalent employees | employee	8,350	8,458	8,198	8,682
Operating Segments | Commercial Bank
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 421	$ 399	$ 1,285	$ 1,206
Noninterest income	383	381	1,007	1,035
Total revenue (TE)	804	780	2,292	2,241
Provision for credit losses	163	32	696	80
Depreciation and amortization expense	35	35	108	100
Other noninterest expense	408	343	1,131	1,051
Income (loss) from continuing operations before income taxes (TE)	198	370	357	1,010
Allocated income taxes and TE adjustments	38	69	32	190
INCOME (LOSS) FROM CONTINUING OPERATIONS	160	301	325	820
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	160	301	325	820
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	160	301	325	820
AVERAGE BALANCES
Loans and leases	62,925	58,215	63,679	57,803
Total assets	72,613	66,549	72,988	65,781
Deposits	51,238	36,204	45,082	35,534
OTHER FINANCIAL DATA
Net loan charge-offs	$ 104	$ 35	$ 201	$ 88
Return on average allocated equity, continuing operations (as a percent)	12.57%	26.18%	8.93%	24.15%
Return on average allocated equity (as a percent)	12.57%	26.18%	8.93%	24.15%
Average full-time equivalent employees | employee	2,112	2,226	2,092	2,285
Other
SUMMARY OF OPERATIONS
Net interest income (TE)	$ (19)	$ (14)	$ (53)	$ (32)
Noninterest income	31	31	98	90
Total revenue (TE)	12	17	45	58
Provision for credit losses	13	120	14	123
Depreciation and amortization expense	35	38	105	112
Other noninterest expense	(12)	(6)	(32)	40
Income (loss) from continuing operations before income taxes (TE)	(24)	(135)	(42)	(217)
Allocated income taxes and TE adjustments	(47)	(51)	(34)	(96)
INCOME (LOSS) FROM CONTINUING OPERATIONS	23	(84)	(8)	(121)
Income (loss) from discontinued operations, net of taxes	4	3	7	6
NET INCOME (LOSS)	27	(81)	(1)	(115)
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	27	(81)	(1)	(115)
AVERAGE BALANCES
Loans and leases	523	981	707	1,009
Total assets	51,310	41,212	44,277	41,014
Deposits	531	1,080	691	1,427
OTHER FINANCIAL DATA
Net loan charge-offs	$ 0	$ 122	$ 1	$ 124
Return on average allocated equity, continuing operations (as a percent)	1.00%	(3.62%)	(0.12%)	(1.87%)
Return on average allocated equity (as a percent)	1.18%	(3.49%)	(0.01%)	(1.78%)
Average full-time equivalent employees | employee	6,635	6,214	6,468	6,250