NONINTEREST INCOME - Components of Revenue from Contracts with Customers (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 258	$ 285	$ 779	$ 863
Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	173	210	509	611
Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	85	75	270	252
Service charges and fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	96	127	298	375
Service charges and fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	71	102	222	298
Service charges and fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	25	25	76	77
Card fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	56	67	160	190
Card fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	49	57	136	162
Card fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	7	10	24	28
Capital markets fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	50	38	163	140
Capital markets fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Capital markets fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	50	38	163	140
Trust and investment services fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	53	50	151	150
Trust and investment services fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	53	50	151	150
Trust and investment services fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Other banking fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3	3	7	8
Other banking fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	1	0	1
Other banking fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 3	$ 2	$ 7	$ 7