BUSINESS SEGMENTS - Financial Information for Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Interest income	$ 256	$ 273	$ 760	$ 860
Interest expense	24	46	77	172
Net Interest Income	232	227	683	688
Provision for credit losses	(2)	27	3	105
Non-interest income	88	80	251	226
Non-interest expense	181	177	542	541
Amortization of intangibles	3	3	9	10
Income tax expense (benefit)	27	17	74	44
Net income (loss)	111	83	306	214
Total assets	39,361	37,441	39,361	37,441	$ 37,354
Total intangibles	2,307	2,319	2,307	2,319
Operating Segments [Member] | Community Banking [Member]
Segment Reporting Information [Line Items]
Interest income	255	273	759	859
Interest expense	20	41	67	155
Net Interest Income	235	232	692	704
Provision for credit losses	(2)	27	2	105
Non-interest income	68	63	191	180
Non-interest expense	164	162	491	495
Amortization of intangibles	3	3	8	9
Income tax expense (benefit)	27	18	75	47
Net income (loss)	111	85	307	228
Total assets	39,238	37,315	39,238	37,315
Total intangibles	2,271	2,282	2,271	2,282
Operating Segments [Member] | Wealth Management [Member]
Segment Reporting Information [Line Items]
Interest income	0	0	0	0
Interest expense	0	0	0	0
Net Interest Income	0	0	0	0
Provision for credit losses	0	0	0	0
Non-interest income	15	12	45	36
Non-interest expense	10	8	30	26
Amortization of intangibles	0	0	0	0
Income tax expense (benefit)	1	1	3	2
Net income (loss)	4	3	12	8
Total assets	43	36	43	36
Total intangibles	9	9	9	9
Operating Segments [Member] | Insurance [Member]
Segment Reporting Information [Line Items]
Interest income	0	0	0	0
Interest expense	0	0	0	0
Net Interest Income	0	0	0	0
Provision for credit losses	0	0	0	0
Non-interest income	6	7	19	18
Non-interest expense	5	5	15	14
Amortization of intangibles	0	0	1	1
Income tax expense (benefit)	0	1	1	1
Net income (loss)	1	1	2	2
Total assets	34	36	34	36
Total intangibles	27	28	27	28
Parent and Other [Member]
Segment Reporting Information [Line Items]
Interest income	1	0	1	1
Interest expense	4	5	10	17
Net Interest Income	(3)	(5)	(9)	(16)
Provision for credit losses	0	0	1	0
Non-interest income	(1)	(2)	(4)	(8)
Non-interest expense	2	2	6	6
Amortization of intangibles	0	0	0	0
Income tax expense (benefit)	(1)	(3)	(5)	(6)
Net income (loss)	(5)	(6)	(15)	(24)
Total assets	46	54	46	54
Total intangibles	$ 0	$ 0	$ 0	$ 0