COMMITMENTS AND CONTINGENCIES - Narrative (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($) counterparty	Dec. 31, 2016 USD ($)	Dec. 31, 2003
Letters of Credit [Abstract]
Letters of credit outstanding	$ 3,000,000	$ 3,000,000
Risk Participation Agreements [Abstract]
Risk participation agreements	$ 19,000,000	19,000,000
Risk participation agreements number of counterparties | counterparty	90
Risk participation agreements, Maximum term	10 years
Commercial real estate loans held for sale | Purchase commitment
Commitments [Abstract]
Unsettled commercial loan trade purchases	$ 186,000,000	127,000,000
Unsettled commercial loan trade sales	$ 170,000,000	177,000,000
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	$ 0	$ 3,000,000
Remaining obligation due	44,000,000
Education
Commitments [Abstract]
Aggregate purchase principal balance	1,200,000,000
Termination fee	1,000,000
Education | Minimum
Commitments [Abstract]
Aggregate purchase principal balance	750,000,000
Education | Maximum
Commitments [Abstract]
Aggregate purchase principal balance	$ 1,500,000,000
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