BUSINESS OPERATING SEGMENTS - Assets by Segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Net interest income	$ 1,160	$ 1,166	$ 2,320	$ 2,326
Noninterest income	590	462	1,087	890
Total revenue	1,750	1,628	3,407	3,216
Noninterest expense	979	951	1,991	1,888
Profit (loss) before provision for credit losses	771	677	1,416	1,328
Provision for credit losses	464	97	1,064	182
Income before income tax expense	307	580	352	1,146
Income tax expense (benefit)	54	127	65	254
NET INCOME	253	453	287	892
Total average assets	179,793	161,489	173,485	160,955
Operating Segments | Consumer Banking
Segment Reporting Information [Line Items]
Net interest income	814	799	1,607	1,587
Noninterest income	428	277	785	524
Total revenue	1,242	1,076	2,392	2,111
Noninterest expense	735	715	1,473	1,415
Profit (loss) before provision for credit losses	507	361	919	696
Provision for credit losses	80	78	177	145
Income before income tax expense	427	283	742	551
Income tax expense (benefit)	107	70	186	136
NET INCOME	320	213	556	415
Total average assets	71,634	65,485	70,024	65,247
Operating Segments | Commercial Banking
Segment Reporting Information [Line Items]
Net interest income	419	371	784	743
Noninterest income	144	149	269	299
Total revenue	563	520	1,053	1,042
Noninterest expense	213	217	434	426
Profit (loss) before provision for credit losses	350	303	619	616
Provision for credit losses	70	25	113	46
Income before income tax expense	280	278	506	570
Income tax expense (benefit)	59	62	106	127
NET INCOME	221	216	400	443
Total average assets	65,280	56,135	62,142	55,884
Other
Segment Reporting Information [Line Items]
Net interest income	(73)	(4)	(71)	(4)
Noninterest income	18	36	33	67
Total revenue	(55)	32	(38)	63
Noninterest expense	31	19	84	47
Profit (loss) before provision for credit losses	(86)	13	(122)	16
Provision for credit losses	314	(6)	774	(9)
Income before income tax expense	(400)	19	(896)	25
Income tax expense (benefit)	(112)	(5)	(227)	(9)
NET INCOME	(288)	24	(669)	34
Total average assets	$ 42,879	$ 39,869	$ 41,319	$ 39,824