BUSINESS SEGMENTS - Financial Information for Segments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Interest income	$ 273	$ 314	$ 860	$ 941
Interest expense	46	84	172	250
Net Interest Income	227	230	688	691
Provision for credit losses	27	12	105	37
Non-interest income	80	80	226	220
Non-interest expense	177	174	541	508
Amortization of intangibles	3	4	10	11
Income tax expense (benefit)	17	17	44	63
Net income (loss)	83	103	214	292
Total assets	37,441	34,329	37,441	34,329	$ 34,615
Total intangibles	2,319	2,333	2,319	2,333
Operating Segments [Member] | Community Banking [Member]
Segment Reporting Information [Line Items]
Interest income	273	314	859	940
Interest expense	41	79	155	236
Net Interest Income	232	235	704	704
Provision for credit losses	27	12	105	37
Non-interest income	63	65	180	177
Non-interest expense	162	158	495	462
Amortization of intangibles	3	3	9	10
Income tax expense (benefit)	18	19	47	66
Net income (loss)	85	108	228	306
Total assets	37,315	34,207	37,315	34,207
Total intangibles	2,282	2,294	2,282	2,294
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	12	12	36	35
Non-interest expense	8	8	26	26
Amortization of intangibles	0	0	0	0
Income tax expense (benefit)	1	1	2	2
Net income (loss)	3	3	8	7
Total assets	36	29	36	29
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	7	5	18	14
Non-interest expense	5	4	14	12
Amortization of intangibles	0	1	1	1
Income tax expense (benefit)	1	0	1	0
Net income (loss)	1	0	2	1
Total assets	36	36	36	36
Total intangibles	28	29	28	29
Parent and Other [Member]
Segment Reporting Information [Line Items]
Interest income	0	0	1	1
Interest expense	5	5	17	14
Net Interest Income	(5)	(5)	(16)	(13)
Provision for credit losses	0	0	0	0
Non-interest income	(2)	(2)	(8)	(6)
Non-interest expense	2	4	6	8
Amortization of intangibles	0	0	0	0
Income tax expense (benefit)	(3)	(3)	(6)	(5)
Net income (loss)	(6)	(8)	(24)	(22)
Total assets	54	57	54	57
Total intangibles	$ 0	$ 0	$ 0	$ 0