NONINTEREST INCOME - Components of Revenue from Contracts with Customers (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 294	$ 280
Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	190	194
Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	104	86
Service charges and fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	118	123
Service charges and fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	92	97
Service charges and fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	26	26
Card fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	55	59
Card fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	45	50
Card fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	10	9
Capital markets fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	65	49
Capital markets fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Capital markets fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	65	49
Trust and investment services fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	53	47
Trust and investment services fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	53	47
Trust and investment services fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Other banking fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3	2
Other banking fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Other banking fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 3	$ 2