BUSINESS OPERATING SEGMENTS - Assets by Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Net interest income	$ 1,145	$ 1,148	$ 3,471	$ 3,360
Noninterest income	493	416	1,383	1,175
Total revenue	1,638	1,564	4,854	4,535
Noninterest expense	973	910	2,861	2,668
Profit (loss) before provision for credit losses	665	654	1,993	1,867
Provision for credit losses	101	78	283	241
Income before income tax expense	564	576	1,710	1,626
Income tax expense (benefit)	115	133	369	370
NET INCOME	449	443	1,341	1,256
Total average assets	162,110	155,624	161,344	153,482
Operating Segments | Consumer Banking
Segment Reporting Information [Line Items]
Net interest income	799	776	2,386	2,268
Noninterest income	336	258	860	708
Total revenue	1,135	1,034	3,246	2,976
Noninterest expense	718	686	2,133	2,000
Profit (loss) before provision for credit losses	417	348	1,113	976
Provision for credit losses	83	71	228	209
Income before income tax expense	334	277	885	767
Income tax expense (benefit)	83	70	219	193
NET INCOME	251	207	666	574
Total average assets	66,365	62,974	65,624	61,857
Operating Segments | Commercial Banking
Segment Reporting Information [Line Items]
Net interest income	360	380	1,103	1,113
Noninterest income	133	140	432	405
Total revenue	493	520	1,535	1,518
Noninterest expense	213	202	639	610
Profit (loss) before provision for credit losses	280	318	896	908
Provision for credit losses	27	14	73	19
Income before income tax expense	253	304	823	889
Income tax expense (benefit)	57	70	184	203
NET INCOME	196	234	639	686
Total average assets	55,614	52,871	55,793	51,820
Other
Segment Reporting Information [Line Items]
Net interest income	(14)	(8)	(18)	(21)
Noninterest income	24	18	91	62
Total revenue	10	10	73	41
Noninterest expense	42	22	89	58
Profit (loss) before provision for credit losses	(32)	(12)	(16)	(17)
Provision for credit losses	(9)	(7)	(18)	13
Income before income tax expense	(23)	(5)	2	(30)
Income tax expense (benefit)	(25)	(7)	(34)	(26)
NET INCOME	2	2	36	(4)
Total average assets	$ 40,131	$ 39,779	$ 39,927	$ 39,805