Line of Business Results - Financial Information of Business Groups (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) employee	Dec. 31, 2016 USD ($) employee	Dec. 31, 2015 USD ($) employee
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 3,830	$ 2,953	$ 2,376
Noninterest income	2,478	2,071	1,880
Total revenue (TE)	6,308	5,024	4,256
Provision for credit losses	229	266	166
Depreciation and amortization expense	384	303	255
Other noninterest expense	3,714	3,453	2,585
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	1,981	1,002	1,250
Allocated income taxes (benefit) and TE adjustments	690	213	331
INCOME (LOSS) FROM CONTINUING OPERATIONS	1,291	789	919
Income (loss) from discontinued operations, net of taxes	7	1	1
NET INCOME (LOSS)	1,298	790	920
Less: Net income (loss) attributable to noncontrolling interests	2	(1)	4
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	1,296	791	916
AVERAGE BALANCES
Loans and leases	86,365	71,148	58,594
Total assets	133,719	112,537	92,026
Deposits	102,946	86,352	70,630
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	3,097	1,934	139
Net loan charge-offs	$ 208	$ 205	$ 142
Return on average allocated equity, continuing operations (as a percentage)	8.47%	6.25%	8.61%
Return on average allocated equity (as a percentage)	8.51%	6.25%	8.62%
Average full-time equivalent employees | employee	18,415	15,700	13,483
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 3,810	$ 2,957	$ 2,365
Noninterest income	2,498	2,089	1,897
Total revenue (TE)	6,308	5,046	4,262
Provision for credit losses	229	266	165
Depreciation and amortization expense	215	140	107
Other noninterest expense	3,686	3,183	2,714
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	2,178	1,457	1,276
Allocated income taxes (benefit) and TE adjustments	587	375	348
INCOME (LOSS) FROM CONTINUING OPERATIONS	1,591	1,082	928
Income (loss) from discontinued operations, net of taxes	0	0	0
NET INCOME (LOSS)	1,591	1,082	928
Less: Net income (loss) attributable to noncontrolling interests	3	0	4
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	1,588	1,082	924
AVERAGE BALANCES
Loans and leases	86,340	71,035	58,551
Total assets	133,033	110,035	91,424
Deposits	102,975	85,869	70,673
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	3,016	1,818	91
Net loan charge-offs	$ 207	$ 204	$ 141
Return on average allocated equity, continuing operations (as a percentage)	20.47%	18.19%	19.25%
Return on average allocated equity (as a percentage)	20.47%	18.19%	19.25%
Average full-time equivalent employees | employee	13,333	11,185	9,634
Operating Segments [Member] | Key Community Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 2,643	$ 1,953	$ 1,487
Noninterest income	1,200	925	788
Total revenue (TE)	3,843	2,878	2,275
Provision for credit losses	209	143	90
Depreciation and amortization expense	116	76	56
Other noninterest expense	2,486	2,077	1,723
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	1,032	582	406
Allocated income taxes (benefit) and TE adjustments	383	217	151
INCOME (LOSS) FROM CONTINUING OPERATIONS	649	365	255
Income (loss) from discontinued operations, net of taxes	0	0	0
NET INCOME (LOSS)	649	365	255
Less: Net income (loss) attributable to noncontrolling interests	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	649	365	255
AVERAGE BALANCES
Loans and leases	47,383	37,620	30,834
Total assets	51,433	40,300	32,948
Deposits	79,669	63,873	51,163
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	2,438	1,478	75
Net loan charge-offs	$ 166	$ 114	$ 92
Return on average allocated equity, continuing operations (as a percentage)	13.59%	10.62%	9.42%
Return on average allocated equity (as a percentage)	13.59%	10.62%	9.42%
Average full-time equivalent employees | employee	10,924	8,936	7,520
Operating Segments [Member] | Key Corporate Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 1,190	$ 1,049	$ 886
Noninterest income	1,147	1,013	926
Total revenue (TE)	2,337	2,062	1,812
Provision for credit losses	20	127	83
Depreciation and amortization expense	96	60	43
Other noninterest expense	1,161	1,071	945
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	1,060	804	741
Allocated income taxes (benefit) and TE adjustments	246	178	196
INCOME (LOSS) FROM CONTINUING OPERATIONS	814	626	545
Income (loss) from discontinued operations, net of taxes	0	0	0
NET INCOME (LOSS)	814	626	545
Less: Net income (loss) attributable to noncontrolling interests	0	(2)	1
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	814	628	544
AVERAGE BALANCES
Loans and leases	37,732	31,929	25,865
Total assets	44,521	37,801	31,541
Deposits	21,318	20,783	19,043
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	559	340	16
Net loan charge-offs	$ 40	$ 83	$ 40
Return on average allocated equity, continuing operations (as a percentage)	28.69%	26.80%	28.91%
Return on average allocated equity (as a percentage)	28.69%	26.80%	28.91%
Average full-time equivalent employees | employee	2,407	2,244	2,100
Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ (23)	$ (45)	$ (8)
Noninterest income	151	151	183
Total revenue (TE)	128	106	175
Provision for credit losses	0	(4)	(8)
Depreciation and amortization expense	3	4	8
Other noninterest expense	39	35	46
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	86	71	129
Allocated income taxes (benefit) and TE adjustments	(42)	(20)	1
INCOME (LOSS) FROM CONTINUING OPERATIONS	128	91	128
Income (loss) from discontinued operations, net of taxes	0	0	0
NET INCOME (LOSS)	128	91	128
Less: Net income (loss) attributable to noncontrolling interests	3	2	3
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	125	89	125
AVERAGE BALANCES
Loans and leases	1,225	1,486	1,852
Total assets	37,079	31,934	26,935
Deposits	1,988	1,213	467
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	19	0	0
Net loan charge-offs	$ 1	$ 7	$ 9
Return on average allocated equity, continuing operations (as a percentage)	86.81%	52.66%	58.96%
Return on average allocated equity (as a percentage)	86.81%	52.66%	58.96%
Average full-time equivalent employees | employee	2	5	14
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 20	$ (4)	$ 11
Noninterest income	(20)	(18)	(17)
Total revenue (TE)	0	(22)	(6)
Provision for credit losses	0	0	1
Depreciation and amortization expense	169	163	148
Other noninterest expense	28	270	(129)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(197)	(455)	(26)
Allocated income taxes (benefit) and TE adjustments	103	(162)	(17)
INCOME (LOSS) FROM CONTINUING OPERATIONS	(300)	(293)	(9)
Income (loss) from discontinued operations, net of taxes	7	1	1
NET INCOME (LOSS)	(293)	(292)	(8)
Less: Net income (loss) attributable to noncontrolling interests	(1)	(1)	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	(292)	(291)	(8)
AVERAGE BALANCES
Loans and leases	25	113	43
Total assets	686	2,502	602
Deposits	(29)	483	(43)
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	81	116	49
Net loan charge-offs	$ 1	$ 1	$ 1
Return on average allocated equity, continuing operations (as a percentage)	(4.00%)	(4.36%)	(0.15%)
Return on average allocated equity (as a percentage)	(3.91%)	(4.34%)	(0.14%)
Average full-time equivalent employees | employee	5,082	4,515	3,849