COMMITMENTS AND CONTINGENCIES - Narrative (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) counterparty	Dec. 31, 2018 USD ($)	Dec. 31, 2003
Risk Participation Agreements [Abstract]
Risk participation agreements number of counterparties | counterparty	83
Risk participation agreements, maximum term	9 years
Commercial real estate loans held for sale | Purchase commitment
Other Commitments [Abstract]
Unsettled commercial loan trade purchases	$ 106	$ 68
Unsettled commercial loan trade sales	$ 114	$ 161
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
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 principal purchase commitment	$ 700
Purchase of Education Loans | Minimum
Other Commitments [Abstract]
Commitment amount	600
Purchase of Education Loans | Maximum
Other Commitments [Abstract]
Commitment amount	$ 1,000