Line of Business Results - Financial Information of Business Groups (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) employee	Dec. 31, 2015 USD ($) employee	Dec. 31, 2014 USD ($) employee
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 2,953	$ 2,376	$ 2,317
Noninterest income	2,071	1,880	1,797
Total revenue (TE)	5,024	4,256	4,114
Provision for credit losses	266	166	57
Depreciation and amortization expense	303	255	260
Other noninterest expense	3,453	2,585	2,501
Income (loss) from continuing operations before income taxes	1,002	1,250	1,296
Allocated income taxes (benefit) and TE adjustments	213	331	350
INCOME (LOSS) FROM CONTINUING OPERATIONS	789	919	946
Income (loss) from discontinued operations, net of taxes	1	1	(39)
NET INCOME (LOSS)	790	920	907
Less: Net income (loss) attributable to noncontrolling interests	(1)	4	7
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	791	916	900
AVERAGE BALANCES
Loans and leases	71,148	58,594	55,679
Total assets	112,537	92,026	87,079
Deposits	86,352	70,630	67,865
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	1,934	139	265
Net loan charge-offs	$ 205	$ 142	$ 113
Return on average allocated equity, continuing operations (as a percentage)	6.25%	8.61%	8.97%
Return on average allocated equity (as a percentage)	6.25%	8.61%	8.59%
Average full-time equivalent employees | employee	15,700	13,483	13,853
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 2,951	$ 2,366	$ 2,312
Noninterest income	2,092	1,897	1,807
Total revenue (TE)	5,043	4,263	4,119
Provision for credit losses	263	165	58
Depreciation and amortization expense	139	107	108
Other noninterest expense	3,167	2,716	2,609
Income (loss) from continuing operations before income taxes	1,474	1,275	1,344
Allocated income taxes (benefit) and TE adjustments	384	346	388
INCOME (LOSS) FROM CONTINUING OPERATIONS	1,090	929	956
Income (loss) from discontinued operations, net of taxes	0	0	0
NET INCOME (LOSS)	1,090	929	956
Less: Net income (loss) attributable to noncontrolling interests	0	4	7
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	1,090	925	949
AVERAGE BALANCES
Loans and leases	71,025	58,551	55,611
Total assets	111,650	91,429	86,424
Deposits	86,159	70,673	67,989
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	1,818	91	147
Net loan charge-offs	$ 204	$ 141	$ 113
Return on average allocated equity, continuing operations (as a percentage)	18.27%	19.34%	20.52%
Return on average allocated equity (as a percentage)	18.27%	19.34%	20.52%
Average full-time equivalent employees | employee	11,186	9,634	9,778
Operating Segments [Member] | Key Community Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 1,947	$ 1,487	$ 1,447
Noninterest income	925	788	769
Total revenue (TE)	2,872	2,275	2,216
Provision for credit losses	147	70	59
Depreciation and amortization expense	75	56	65
Other noninterest expense	2,056	1,729	1,696
Income (loss) from continuing operations before income taxes	594	420	396
Allocated income taxes (benefit) and TE adjustments	221	156	147
INCOME (LOSS) FROM CONTINUING OPERATIONS	373	264	249
Income (loss) from discontinued operations, net of taxes	0	0	0
NET INCOME (LOSS)	373	264	249
Less: Net income (loss) attributable to noncontrolling interests	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	373	264	249
AVERAGE BALANCES
Loans and leases	37,613	30,834	30,105
Total assets	40,284	32,948	32,241
Deposits	63,895	51,163	50,316
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	1,478	75	45
Net loan charge-offs	$ 114	$ 92	$ 117
Return on average allocated equity, continuing operations (as a percentage)	10.87%	9.80%	9.14%
Return on average allocated equity (as a percentage)	10.87%	9.80%	9.14%
Average full-time equivalent employees | employee	8,928	7,520	7,761
Operating Segments [Member] | Key Corporate Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 1,048	$ 887	$ 842
Noninterest income	1,014	926	807
Total revenue (TE)	2,062	1,813	1,649
Provision for credit losses	120	103	14
Depreciation and amortization expense	60	43	31
Other noninterest expense	1,067	940	843
Income (loss) from continuing operations before income taxes	815	727	761
Allocated income taxes (benefit) and TE adjustments	183	190	215
INCOME (LOSS) FROM CONTINUING OPERATIONS	632	537	546
Income (loss) from discontinued operations, net of taxes	0	0	0
NET INCOME (LOSS)	632	537	546
Less: Net income (loss) attributable to noncontrolling interests	(2)	1	2
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	634	536	544
AVERAGE BALANCES
Loans and leases	31,935	25,865	22,978
Total assets	37,801	31,546	28,070
Deposits	20,783	19,043	17,094
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	340	16	102
Net loan charge-offs	$ 83	$ 40	$ (18)
Return on average allocated equity, continuing operations (as a percentage)	26.85%	28.57%	33.56%
Return on average allocated equity (as a percentage)	26.85%	28.57%	33.56%
Average full-time equivalent employees | employee	2,248	2,100	1,975
Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ (44)	$ (8)	$ 23
Noninterest income	153	183	231
Total revenue (TE)	109	175	254
Provision for credit losses	(4)	(8)	(15)
Depreciation and amortization expense	4	8	12
Other noninterest expense	44	47	70
Income (loss) from continuing operations before income taxes	65	128	187
Allocated income taxes (benefit) and TE adjustments	(20)	0	26
INCOME (LOSS) FROM CONTINUING OPERATIONS	85	128	161
Income (loss) from discontinued operations, net of taxes	0	0	0
NET INCOME (LOSS)	85	128	161
Less: Net income (loss) attributable to noncontrolling interests	2	3	5
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	83	125	156
AVERAGE BALANCES
Loans and leases	1,477	1,852	2,528
Total assets	33,565	26,935	26,113
Deposits	1,481	467	579
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	0	0	0
Net loan charge-offs	$ 7	$ 9	$ 14
Return on average allocated equity, continuing operations (as a percentage)	47.43%	58.96%	55.71%
Return on average allocated equity (as a percentage)	47.43%	58.96%	55.71%
Average full-time equivalent employees | employee	10	14	42
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 2	$ 10	$ 5
Noninterest income	(21)	(17)	(10)
Total revenue (TE)	(19)	(7)	(5)
Provision for credit losses	3	1	(1)
Depreciation and amortization expense	164	148	152
Other noninterest expense	286	(131)	(108)
Income (loss) from continuing operations before income taxes	(472)	(25)	(48)
Allocated income taxes (benefit) and TE adjustments	(171)	(15)	(38)
INCOME (LOSS) FROM CONTINUING OPERATIONS	(301)	(10)	(10)
Income (loss) from discontinued operations, net of taxes	1	1	(39)
NET INCOME (LOSS)	(300)	(9)	(49)
Less: Net income (loss) attributable to noncontrolling interests	(1)	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	(299)	(9)	(49)
AVERAGE BALANCES
Loans and leases	123	43	68
Total assets	887	597	655
Deposits	193	(43)	(124)
OTHER FINANCIAL DATA
Expenditures for additions to long-lived assets	116	48	118
Net loan charge-offs	$ 1	$ 1	$ 0
Return on average allocated equity, continuing operations (as a percentage)	(4.49%)	(0.17%)	(0.17%)
Return on average allocated equity (as a percentage)	(4.48%)	(0.15%)	(0.84%)
Average full-time equivalent employees | employee	4,514	3,849	4,075