FAIR VALUE MEASUREMENTS - Schedule of Fair Value Measurements on a Nonrecurring Basis (Details) - Nonrecurring measurement basis - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired collateral-dependent loans	$ 371	$ 338
MSRs	224	243
Foreclosed assets	29	29
Leased assets	91	92
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired collateral-dependent loans	0	0
MSRs	0	0
Foreclosed assets	0	0
Leased assets	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired collateral-dependent loans	371	338
MSRs	0	0
Foreclosed assets	29	29
Leased assets	91	92
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired collateral-dependent loans	0	0
MSRs	224	243
Foreclosed assets	0	0
Leased assets	$ 0	$ 0