BUSINESS SEGMENTS - Financial Information for Segments (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Interest income	$ 281	$ 317	$ 587	$ 627
Interest expense	52	87	126	166
Net Interest Income	229	230	461	461
Provision for credit losses	30	11	78	25
Non-interest income	77	75	146	140
Non-interest expense	172	172	364	334
Amortization of intangibles	4	3	7	7
Income tax expense (benefit)	16	24	27	46
Net income (loss)	84	95	131	189
Total assets	37,721	33,903	37,721	33,903	$ 34,615
Total intangibles	2,323	2,336	2,323	2,336
Operating Segments [Member] | Community Banking [Member]
Segment Reporting Information [Line Items]
Interest income	280	316	586	626
Interest expense	46	81	114	157
Net Interest Income	234	235	472	469
Provision for credit losses	30	11	78	25
Non-interest income	65	61	117	112
Non-interest expense	157	157	333	304
Amortization of intangibles	3	3	6	7
Income tax expense (benefit)	17	24	29	47
Net income (loss)	92	101	143	198
Total assets	37,598	33,785	37,598	33,785
Total intangibles	2,285	2,297	2,285	2,297
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	11	12	24	23
Non-interest expense	8	9	18	18
Amortization of intangibles	0	0	0	0
Income tax expense (benefit)	0	1	1	1
Net income (loss)	3	2	5	4
Total assets	33	27	33	27
Total intangibles	9	10	9	10
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	5	4	11	9
Non-interest expense	4	4	9	8
Amortization of intangibles	1	0	1	0
Income tax expense (benefit)	0	0	0	0
Net income (loss)	0	0	1	1
Total assets	36	35	36	35
Total intangibles	29	29	29	29
Parent and Other [Member]
Segment Reporting Information [Line Items]
Interest income	1	1	1	1
Interest expense	6	6	12	9
Net Interest Income	(5)	(5)	(11)	(8)
Provision for credit losses	0	0	0	0
Non-interest income	(4)	(2)	(6)	(4)
Non-interest expense	3	2	4	4
Amortization of intangibles	0	0	0	0
Income tax expense (benefit)	(1)	(1)	(3)	(2)
Net income (loss)	(11)	(8)	(18)	(14)
Total assets	54	56	54	56
Total intangibles	$ 0	$ 0	$ 0	$ 0