COMMITMENTS AND CONTINGENCIES - Narrative (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) counterparty	Dec. 31, 2017 USD ($)	Dec. 31, 2003
Letters of Credit [Abstract]
Letters of credit outstanding, amount	$ 2	$ 3
Risk Participation Agreements [Abstract]
Risk participation agreements	$ 14	16
Risk participation agreements number of counterparties | counterparty	87
Risk participation agreements, Maximum term	10 years
Commercial real estate loans held for sale | Purchase commitment
Other Commitments [Abstract]
Unsettled commercial loan trade purchases	$ 87	65
Unsettled commercial loan trade sales	$ 121	132
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	$ 4	$ 3
Remaining obligation due	$ 37
Financial standby letters of credit
Letters of Credit [Abstract]
Letters of credit outstanding	10 years
Commercial letters of credit
Letters of Credit [Abstract]
Letters of credit outstanding	1 year
Purchase Of Education Loans
Other Commitments [Abstract]
Remaining maximum purchase commitment	$ 188
Termination fee	1
Purchase Of Education Loans | Minimum
Other Commitments [Abstract]
Commitment amount	425
Purchase Of Education Loans | Maximum
Other Commitments [Abstract]
Commitment amount	$ 700