FAIR VALUE MEASUREMENTS - Schedule of Gain (Loss) on Assets and Liabilities Measured on Nonrecurring Basis Included in Earnings (Details) - Nonrecurring measurement basis - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Impaired collateral-dependent loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain (loss) included in earnings on assets measured on a nonrecurring basis	$ (4)	$ (2)
MSRs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain (loss) included in earnings on assets measured on a nonrecurring basis	0	3
Foreclosed assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain (loss) included in earnings on assets measured on a nonrecurring basis	0	(1)
Leased assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain (loss) included in earnings on assets measured on a nonrecurring basis	$ (3)	$ 0