COMMITMENTS AND CONTINGENCIES - Narrative (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) counterparty	Dec. 31, 2018 USD ($)	Dec. 31, 2003
Risk Participation Agreements [Abstract]
Risk participation agreements number of counterparties | counterparty	86
Risk participation agreements, Maximum term	9 years
Commercial real estate loans held for sale | Purchase commitment
Other Commitments [Abstract]
Unsettled commercial loan trade purchases	$ 166	$ 68
Unsettled commercial loan trade sales	$ 234	$ 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]
Other Commitment, Remaining Maximum Amount Committed	$ 799
Purchase Of Education Loans | Minimum
Other Commitments [Abstract]
Commitment amount	600
Purchase Of Education Loans | Maximum
Other Commitments [Abstract]
Commitment amount	$ 1,000