BUSINESS OPERATING SEGMENTS - Assets by Segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Net interest income	$ 1,124	$ 1,160	$ 2,241	$ 2,320
Noninterest income	485	590	1,027	1,087
Total revenue	1,609	1,750	3,268	3,407
Noninterest expense	991	979	2,009	1,991
Profit (loss) before provision for credit losses	618	771	1,259	1,416
Provision for credit losses	(213)	464	(353)	1,064
Income before income tax expense	831	307	1,612	352
Income tax expense (benefit)	183	54	353	65
NET INCOME	648	253	1,259	287
Total average assets	184,456	179,793	183,518	173,485
Operating Segments
Segment Reporting Information [Line Items]
Noninterest income	485	590	1,027	1,087
Operating Segments | Consumer Banking
Segment Reporting Information [Line Items]
Net interest income	897	814	1,760	1,607
Noninterest income	283	428	634	785
Total revenue	1,180	1,242	2,394	2,392
Noninterest expense	751	735	1,501	1,473
Profit (loss) before provision for credit losses	429	507	893	919
Provision for credit losses	45	80	104	177
Income before income tax expense	384	427	789	742
Income tax expense (benefit)	98	107	201	186
NET INCOME	286	320	588	556
Total average assets	75,600	71,634	75,443	70,024
Operating Segments | Commercial Banking
Segment Reporting Information [Line Items]
Net interest income	419	419	840	784
Noninterest income	178	144	348	269
Total revenue	597	563	1,188	1,053
Noninterest expense	226	213	453	434
Profit (loss) before provision for credit losses	371	350	735	619
Provision for credit losses	34	70	135	113
Income before income tax expense	337	280	600	506
Income tax expense (benefit)	72	59	124	106
NET INCOME	265	221	476	400
Total average assets	57,527	65,280	57,632	62,142
Other
Segment Reporting Information [Line Items]
Net interest income	(192)	(73)	(359)	(71)
Total revenue	(168)	(55)	(314)	(38)
Noninterest expense	14	31	55	84
Profit (loss) before provision for credit losses	(182)	(86)	(369)	(122)
Provision for credit losses	(292)	314	(592)	774
Income before income tax expense	110	(400)	223	(896)
Income tax expense (benefit)	13	(112)	28	(227)
NET INCOME	97	(288)	195	(669)
Total average assets	$ 51,329	$ 42,879	$ 50,443	$ 41,319