NONINTEREST INCOME - Components of Revenue from Contracts with Customers (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 283	$ 548
Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	194	384
Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	89	164
Service charges and fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	127	251
Service charges and fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	100	198
Service charges and fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	27	53
Card fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	60	121
Card fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	51	103
Card fees | Commercial Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	9	18
Capital markets fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	51	88
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	51	88
Trust and investment services fees
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	43	83
Trust and investment services fees | Consumer Banking
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	43	83
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	2	5
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	$ 2	$ 5