FAIR VALUE MEASUREMENTS - Schedule of Fair Value Measurements on a Nonrecurring Basis (Details) - Nonrecurring measurement basis - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired collateral-dependent loans	$ 300	$ 338
MSRs	193	243
Foreclosed assets	27	29
Leased assets before 2016-02 adoption		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 before 2016-02 adoption		0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired collateral-dependent loans	300	338
MSRs	0	0
Foreclosed assets	27	29
Leased assets before 2016-02 adoption		92
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired collateral-dependent loans	0	0
MSRs	193	243
Foreclosed assets	0	0
Leased assets before 2016-02 adoption		$ 0
Assets Held Under Finance Leases
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Leased assets	61
Assets Held Under Finance Leases | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Leased assets	0
Assets Held Under Finance Leases | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Leased assets	61
Assets Held Under Finance Leases | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Leased assets	$ 0