COMMITMENTS AND CONTINGENCIES - Narrative (Details) $ in Millions		6 Months Ended	12 Months Ended
	Jan. 07, 2016 USD ($)	Jun. 30, 2016 USD ($) counterparty	Dec. 31, 2015 USD ($)	Dec. 31, 2003
Letters of Credit [Abstract]
Letters of credit outstanding		$ 3	$ 3
Risk Participation Agreements [Abstract]
Risk participation agreements		$ 64	26
Risk participation agreements number of counterparties | counterparty		89
Risk participation agreements, Maximum term		10 years
Commercial real estate loans held for sale | Purchase commitment
Commitments [Abstract]
Unsettled commercial loan trade purchases		$ 111
Unsettled commercial loan trade sales		$ 108
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		$ 1	$ 3
Remaining obligation due		46
Automobile | Minimum
Commitments [Abstract]
Purchase commitment, due after next twelve months		50
Automobile | Maximum
Commitments [Abstract]
Purchase commitment, due after next twelve months		$ 200
Student
Commitments [Abstract]
Termination fee	$ 1
Student | Minimum
Commitments [Abstract]
Purchase commitment, due in next twelve months	125
Aggregate purchase principal balance	500
Student | Maximum
Commitments [Abstract]
Aggregate purchase principal balance	$ 1,000
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