NONINTEREST INCOME - Components of Revenue from Contracts with Customers (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 227	$ 298	$ 521	$ 578
Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	146	207	336	401
Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	81	91	185	177
Service charges and fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	84	125	202	248
Service charges and fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	59	99	151	196
Service charges and fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	25	26	51	52
Card fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	49	64	104	123
Card fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	42	55	87	105
Card fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	7	9	17	18
Capital markets fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	48	53	113	102
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	48	53	113	102
Trust and investment services fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	45	53	98	100
Trust and investment services fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	45	53	98	100
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	1	3	4	5
Other banking fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Other banking fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 1	$ 3	$ 4	$ 5