Business Segment Reporting - Schedule of Segment Reporting Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) employee	Mar. 31, 2018 USD ($) employee
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 985	$ 952
Noninterest income	536	601
Total revenue (TE)	1,521	1,553
Provision for credit losses	62	61
Depreciation and amortization expense	88	103
Other noninterest expense	875	903
Income (loss) from continuing operations before income taxes (TE)	496	486
Allocated income taxes and TE adjustments	90	70
INCOME (LOSS) FROM CONTINUING OPERATIONS	406	416
Income (loss) from discontinued operations, net of taxes	1	2
NET INCOME (LOSS)	407	418
Less: Net income (loss) attributable to noncontrolling interests	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	407	418
AVERAGE BALANCES
Loans and leases	89,649	86,927
Total assets	140,120	134,915
Deposits	107,576	102,556
OTHER FINANCIAL DATA
Net loan charge-offs	$ 64	$ 54
Return on average allocated equity, continuing operations	10.49%	11.33%
Return on average allocated equity	10.51%	11.39%
Average full-time equivalent employees | employee	17,554	18,540
Operating Segments [Member] | Consumer Bank [Member]
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 594	$ 553
Noninterest income	214	229
Total revenue (TE)	808	782
Provision for credit losses	45	34
Depreciation and amortization expense	23	27
Other noninterest expense	524	549
Income (loss) from continuing operations before income taxes (TE)	216	172
Allocated income taxes and TE adjustments	52	41
INCOME (LOSS) FROM CONTINUING OPERATIONS	164	131
Income (loss) from discontinued operations, net of taxes	0	0
NET INCOME (LOSS)	164	131
Less: Net income (loss) attributable to noncontrolling interests	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	164	131
AVERAGE BALANCES
Loans and leases	31,403	31,647
Total assets	34,814	34,802
Deposits	71,289	67,421
OTHER FINANCIAL DATA
Net loan charge-offs	$ 34	$ 35
Return on average allocated equity, continuing operations	19.83%	16.41%
Return on average allocated equity	19.83%	16.41%
Average full-time equivalent employees | employee	9,631	10,106
Operating Segments [Member] | Commercial Bank [Member]
SUMMARY OF OPERATIONS
Net interest income (TE)	$ 399	$ 405
Noninterest income	300	325
Total revenue (TE)	699	730
Provision for credit losses	15	28
Depreciation and amortization expense	29	35
Other noninterest expense	338	346
Income (loss) from continuing operations before income taxes (TE)	317	321
Allocated income taxes and TE adjustments	64	45
INCOME (LOSS) FROM CONTINUING OPERATIONS	253	276
Income (loss) from discontinued operations, net of taxes	0	0
NET INCOME (LOSS)	253	276
Less: Net income (loss) attributable to noncontrolling interests	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	253	276
AVERAGE BALANCES
Loans and leases	57,210	54,131
Total assets	64,817	61,750
Deposits	34,418	32,794
OTHER FINANCIAL DATA
Net loan charge-offs	$ 30	$ 19
Return on average allocated equity, continuing operations	23.66%	25.31%
Return on average allocated equity	23.66%	25.31%
Average full-time equivalent employees | employee	2,360	2,457
Other [Member]
SUMMARY OF OPERATIONS
Net interest income (TE)	$ (8)	$ (6)
Noninterest income	22	47
Total revenue (TE)	14	41
Provision for credit losses	2	(1)
Depreciation and amortization expense	36	41
Other noninterest expense	13	8
Income (loss) from continuing operations before income taxes (TE)	(37)	(7)
Allocated income taxes and TE adjustments	(26)	(16)
INCOME (LOSS) FROM CONTINUING OPERATIONS	(11)	9
Income (loss) from discontinued operations, net of taxes	1	2
NET INCOME (LOSS)	(10)	11
Less: Net income (loss) attributable to noncontrolling interests	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	(10)	11
AVERAGE BALANCES
Loans and leases	1,036	1,149
Total assets	40,489	38,363
Deposits	1,869	2,341
OTHER FINANCIAL DATA
Net loan charge-offs	$ 0	$ 0
Return on average allocated equity, continuing operations	(0.56%)	0.50%
Return on average allocated equity	(0.51%)	0.62%
Average full-time equivalent employees | employee	5,563	5,977