BUSINESS OPERATING SEGMENTS - Assets by Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Net interest income	$ 1,137	$ 1,145	$ 3,457	$ 3,471
Noninterest income	654	493	1,741	1,383
Total revenue	1,791	1,638	5,198	4,854
Noninterest expense	988	973	2,979	2,861
Profit (loss) before provision for credit losses	803	665	2,219	1,993
Provision for credit losses	428	101	1,492	283
Income before income tax expense	375	564	727	1,710
Income tax expense (benefit)	61	115	126	369
NET INCOME	314	449	601	1,341
Total average assets	177,675	162,110	174,892	161,344
Operating Segments | Consumer Banking
Segment Reporting Information [Line Items]
Net interest income	845	799	2,452	2,386
Noninterest income	495	336	1,280	860
Total revenue	1,340	1,135	3,732	3,246
Noninterest expense	742	718	2,215	2,133
Profit (loss) before provision for credit losses	598	417	1,517	1,113
Provision for credit losses	55	83	232	228
Income before income tax expense	543	334	1,285	885
Income tax expense (benefit)	136	83	322	219
NET INCOME	407	251	963	666
Total average assets	73,605	66,365	71,227	65,624
Operating Segments | Commercial Banking
Segment Reporting Information [Line Items]
Net interest income	421	360	1,205	1,103
Noninterest income	144	133	413	432
Total revenue	565	493	1,618	1,535
Noninterest expense	210	213	644	639
Profit (loss) before provision for credit losses	355	280	974	896
Provision for credit losses	161	27	274	73
Income before income tax expense	194	253	700	823
Income tax expense (benefit)	41	57	147	184
NET INCOME	153	196	553	639
Total average assets	60,889	55,614	61,722	55,793
Other
Segment Reporting Information [Line Items]
Net interest income	(129)	(14)	(200)	(18)
Noninterest income	15	24	48	91
Total revenue	(114)	10	(152)	73
Noninterest expense	36	42	120	89
Profit (loss) before provision for credit losses	(150)	(32)	(272)	(16)
Provision for credit losses	212	(9)	986	(18)
Income before income tax expense	(362)	(23)	(1,258)	2
Income tax expense (benefit)	(116)	(25)	(343)	(34)
NET INCOME	(246)	2	(915)	36
Total average assets	$ 43,181	$ 40,131	$ 41,943	$ 39,927