COMMITMENTS AND CONTINGENCIES - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) counterparty	Dec. 31, 2016 USD ($)	Dec. 31, 2003
Letters of Credit [Abstract]
Letters of credit outstanding	$ 3	$ 3
Risk Participation Agreements [Abstract]
Risk participation agreements	$ 16	19
Risk participation agreements number of counterparties | counterparty	88
Risk participation agreements, Maximum term	9 years
Commercial loans held for sale | Purchase commitment
Commitments [Abstract]
Unsettled commercial loan trade purchases	$ 65	127
Unsettled commercial loan trade sales	$ 132	177
Minimum
Risk Participation Agreements [Abstract]
Risk participation agreements, Average term	1 year
Maximum
Risk Participation Agreements [Abstract]
Risk participation agreements, Average term	5 years
Marketing rights
Marketing Rights [Abstract]
Commitment period			25 years
Payments made	$ 3	$ 3
Remaining obligation due	$ 41
Financial standby letters of credit
Letters of Credit [Abstract]
Letters of credit terms	10 years
Commercial letters of credit
Letters of Credit [Abstract]
Letters of credit terms	1 year