Line of Business Results - Financial Information of Business Groups (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) employee	Sep. 30, 2016 USD ($) employee	Sep. 30, 2017 USD ($) employee	Sep. 30, 2016 USD ($) employee
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 962	$ 788	$ 2,878	$ 2,005
Noninterest income	592	549	1,822	1,453
Total revenue (TE)	1,554	1,337	4,700	3,458
Provision for credit losses	51	59	180	200
Depreciation and amortization expense	97	77	283	203
Other noninterest expense	895	1,005	2,717	2,333
Income (loss) from continuing operations before income taxes	511	196	1,520	722
Allocated income taxes and TE adjustments	148	24	425	165
INCOME (LOSS) FROM CONTINUING OPERATIONS	363	172	1,095	557
Income (loss) from discontinued operations, net of taxes	1	1	6	5
NET INCOME (LOSS)	364	173	1,101	562
Less: Net income (loss) attributable to noncontrolling interests	0	1	1	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	364	172	1,100	562
AVERAGE BALANCES
Loans and leases	86,814	77,697	86,485	66,375
Total assets	134,356	123,469	133,202	105,187
Deposits	103,105	94,918	102,659	80,194
OTHER FINANCIAL DATA
Net loan charge-offs	$ 32	$ 44	$ 157	$ 133
Return on average allocated equity	9.45%	5.02%	9.62%	6.26%
Return on average allocated equity	9.48%	5.05%	9.67%	6.31%
Average full-time equivalent employees | employee	18,548	17,079	18,427	14,642
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 954	$ 798	$ 2,865	$ 2,007
Noninterest income	595	557	1,837	1,469
Total revenue (TE)	1,549	1,355	4,702	3,476
Provision for credit losses	50	59	180	200
Depreciation and amortization expense	56	33	156	87
Other noninterest expense	901	876	2,701	2,218
Income (loss) from continuing operations before income taxes	542	387	1,665	971
Allocated income taxes and TE adjustments	169	114	497	253
INCOME (LOSS) FROM CONTINUING OPERATIONS	373	273	1,168	718
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	373	273	1,168	718
Less: Net income (loss) attributable to noncontrolling interests	(1)	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	374	273	1,168	718
AVERAGE BALANCES
Loans and leases	86,810	77,551	86,462	66,283
Total assets	133,794	116,130	132,436	102,423
Deposits	103,132	93,169	102,705	79,615
OTHER FINANCIAL DATA
Net loan charge-offs	$ 32	$ 43	$ 156	$ 134
Return on average allocated equity	19.14%	17.82%	20.10%	17.97%
Return on average allocated equity	19.14%	17.82%	20.10%	17.97%
Average full-time equivalent employees | employee	13,494	12,139	13,317	10,382
Operating Segments [Member] | Key Community Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 670	$ 533	$ 1,973	$ 1,324
Noninterest income	289	250	901	652
Total revenue (TE)	959	783	2,874	1,976
Provision for credit losses	59	39	152	92
Depreciation and amortization expense	30	17	86	42
Other noninterest expense	613	573	1,835	1,429
Income (loss) from continuing operations before income taxes	257	154	801	413
Allocated income taxes and TE adjustments	96	57	298	154
INCOME (LOSS) FROM CONTINUING OPERATIONS	161	97	503	259
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	161	97	503	259
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	161	97	503	259
AVERAGE BALANCES
Loans and leases	47,595	41,548	47,376	34,450
Total assets	51,708	44,218	51,421	36,707
Deposits	79,563	69,397	79,438	58,704
OTHER FINANCIAL DATA
Net loan charge-offs	$ 41	$ 31	$ 131	$ 72
Return on average allocated equity	13.27%	10.84%	14.13%	11.55%
Return on average allocated equity	13.27%	10.84%	14.13%	11.55%
Average full-time equivalent employees | employee	11,032	9,805	10,912	8,177
Operating Segments [Member] | Key Corporate Bank [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 291	$ 278	$ 907	$ 716
Noninterest income	269	278	827	716
Total revenue (TE)	560	556	1,734	1,432
Provision for credit losses	(11)	23	26	110
Depreciation and amortization expense	25	15	68	41
Other noninterest expense	278	295	836	764
Income (loss) from continuing operations before income taxes	268	223	804	517
Allocated income taxes and TE adjustments	78	63	212	114
INCOME (LOSS) FROM CONTINUING OPERATIONS	190	160	592	403
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	190	160	592	403
Less: Net income (loss) attributable to noncontrolling interests	0	0	(1)	(1)
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	190	160	593	404
AVERAGE BALANCES
Loans and leases	38,040	34,561	37,823	30,312
Total assets	45,276	40,584	44,526	35,984
Deposits	21,559	22,708	21,237	19,980
OTHER FINANCIAL DATA
Net loan charge-offs	$ (9)	$ 12	$ 24	$ 57
Return on average allocated equity	26.92%	26.89%	27.69%	24.87%
Return on average allocated equity	26.92%	26.89%	27.69%	24.87%
Average full-time equivalent employees | employee	2,460	2,330	2,403	2,199
Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ (7)	$ (13)	$ (15)	$ (33)
Noninterest income	37	29	109	101
Total revenue (TE)	30	16	94	68
Provision for credit losses	2	(3)	2	(2)
Depreciation and amortization expense	1	1	2	4
Other noninterest expense	10	8	30	25
Income (loss) from continuing operations before income taxes	17	10	60	41
Allocated income taxes and TE adjustments	(5)	(6)	(13)	(15)
INCOME (LOSS) FROM CONTINUING OPERATIONS	22	16	73	56
Income (loss) from discontinued operations, net of taxes	0	0	0	0
NET INCOME (LOSS)	22	16	73	56
Less: Net income (loss) attributable to noncontrolling interests	(1)	0	1	1
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	23	16	72	55
AVERAGE BALANCES
Loans and leases	1,175	1,442	1,263	1,521
Total assets	36,810	31,328	36,489	29,732
Deposits	2,010	1,064	2,030	931
OTHER FINANCIAL DATA
Net loan charge-offs	$ 0	$ 0	$ 1	$ 5
Return on average allocated equity	65.65%	38.34%	65.93%	42.71%
Return on average allocated equity	65.65%	38.34%	65.93%	42.71%
Average full-time equivalent employees | employee	2	4	2	6
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net interest income (TE)	$ 8	$ (10)	$ 13	$ (2)
Noninterest income	(3)	(8)	(15)	(16)
Total revenue (TE)	5	(18)	(2)	(18)
Provision for credit losses	1	0	0	0
Depreciation and amortization expense	41	44	127	116
Other noninterest expense	(6)	129	16	115
Income (loss) from continuing operations before income taxes	(31)	(191)	(145)	(249)
Allocated income taxes and TE adjustments	(21)	(90)	(72)	(88)
INCOME (LOSS) FROM CONTINUING OPERATIONS	(10)	(101)	(73)	(161)
Income (loss) from discontinued operations, net of taxes	1	1	6	5
NET INCOME (LOSS)	(9)	(100)	(67)	(156)
Less: Net income (loss) attributable to noncontrolling interests	1	1	1	0
NET INCOME (LOSS) ATTRIBUTABLE TO KEY	(10)	(101)	(68)	(156)
AVERAGE BALANCES
Loans and leases	4	146	23	92
Total assets	562	7,339	766	2,764
Deposits	(27)	1,749	(46)	579
OTHER FINANCIAL DATA
Net loan charge-offs	$ 0	$ 1	$ 1	$ (1)
Return on average allocated equity	(0.58%)	(5.44%)	(1.33%)	(3.28%)
Return on average allocated equity	(0.53%)	(5.39%)	(1.22%)	(3.18%)
Average full-time equivalent employees | employee	5,054	4,940	5,110	4,260