COMMITMENTS AND CONTINGENCIES - Narrative (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) counterparty	Dec. 31, 2016 USD ($)	Dec. 31, 2003
Letters of Credit [Abstract]
Letters of credit outstanding, amount	$ 3	$ 3
Risk Participation Agreements [Abstract]
Risk participation agreements	$ 20	19
Risk participation agreements number of counterparties | counterparty	96
Risk participation agreements, Maximum term	9 years
Commercial real estate loans held for sale | Purchase commitment
Other Commitments [Abstract]
Unsettled commercial loan trade purchases	$ 241	127
Unsettled commercial loan trade sales	$ 299	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
Education
Other Commitments [Abstract]
Aggregate purchase principal balance	$ 375
Extension period	1 year
Termination fee	$ 1
Education | Minimum
Other Commitments [Abstract]
Aggregate purchase principal balance	750
Education | Maximum
Other Commitments [Abstract]
Aggregate purchase principal balance	$ 1,500
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