FAIR VALUE MEASUREMENTS - Schedule of Fair Value Measurements on a Nonrecurring Basis (Details) - Nonrecurring measurement basis - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired collateral-dependent loans	$ 408	$ 393
MSRs	254	218
Foreclosed assets	25	31
Leased assets	108	112
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	408	393
MSRs	0	0
Foreclosed assets	25	31
Leased assets	108	112
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired collateral-dependent loans	0	0
MSRs	254	218
Foreclosed assets	0	0
Leased assets	$ 0	$ 0