BUSINESS OPERATING SEGMENTS - Assets by Segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Net interest income	$ 1,160	$ 1,160
Noninterest income	497	428
Total revenue	1,657	1,588
Noninterest expense	1,012	937
Profit (loss) before provision for credit losses	645	651
Provision for credit losses	600	85
Income before income tax expense	45	566
Income tax expense (benefit)	11	127
NET INCOME	34	439
Total average assets	167,177	160,415
Operating Segments | Consumer Banking
Segment Reporting Information [Line Items]
Net interest income	793	788
Noninterest income	357	247
Total revenue	1,150	1,035
Noninterest expense	738	700
Profit (loss) before provision for credit losses	412	335
Provision for credit losses	97	67
Income before income tax expense	315	268
Income tax expense (benefit)	79	66
NET INCOME	236	202
Total average assets	68,415	65,007
Operating Segments | Commercial Banking
Segment Reporting Information [Line Items]
Net interest income	365	372
Noninterest income	125	150
Total revenue	490	522
Noninterest expense	221	209
Profit (loss) before provision for credit losses	269	313
Provision for credit losses	43	21
Income before income tax expense	226	292
Income tax expense (benefit)	47	65
NET INCOME	179	227
Total average assets	59,005	55,630
Other
Segment Reporting Information [Line Items]
Net interest income	2	0
Noninterest income	15	31
Total revenue	17	31
Noninterest expense	53	28
Profit (loss) before provision for credit losses	(36)	3
Provision for credit losses	460	(3)
Income before income tax expense	(496)	6
Income tax expense (benefit)	(115)	(4)
NET INCOME	(381)	10
Total average assets	$ 39,757	$ 39,778