ALLOWANCE FOR CREDIT LOSSES - Schedule of Allowance for Credit Losses for HTM Debt Securities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of period	$ 0
Gross credit losses	0
Gross recoveries	0
Net credit losses	0
Net reserve builds (releases)	6
Net specific reserve builds (releases)	0
Total provision for credit losses on HTM debt securities	6	$ 0
Other, net	0
Initial allowance for credit losses on newly purchased credit deteriorated assets during the period	0
Allowance for credit losses on HTM debt securities at end of period	76
Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of period	70
U.S. Treasury and federal agency securities
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of period	0
Gross credit losses	0
Gross recoveries	0
Net credit losses	0
Net reserve builds (releases)	0
Net specific reserve builds (releases)	0
Total provision for credit losses on HTM debt securities	0
Other, net	0
Initial allowance for credit losses on newly purchased credit deteriorated assets during the period	0
Allowance for credit losses on HTM debt securities at end of period	0
U.S. Treasury and federal agency securities | Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of period	0
State and municipal securities
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of period	0
Gross credit losses	0
Gross recoveries	0
Net credit losses	0
Net reserve builds (releases)	5
Net specific reserve builds (releases)	0
Total provision for credit losses on HTM debt securities	5
Other, net	0
Initial allowance for credit losses on newly purchased credit deteriorated assets during the period	0
Allowance for credit losses on HTM debt securities at end of period	66
State and municipal securities | Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of period	61
Foreign government
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of period	0
Gross credit losses	0
Gross recoveries	0
Net credit losses	0
Net reserve builds (releases)	0
Net specific reserve builds (releases)	0
Total provision for credit losses on HTM debt securities	0
Other, net	0
Initial allowance for credit losses on newly purchased credit deteriorated assets during the period	0
Allowance for credit losses on HTM debt securities at end of period	4
Foreign government | Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of period	4
Asset-backed securities
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of period	0
Gross credit losses	0
Gross recoveries	0
Net credit losses	0
Net reserve builds (releases)	1
Net specific reserve builds (releases)	0
Total provision for credit losses on HTM debt securities	1
Other, net	0
Initial allowance for credit losses on newly purchased credit deteriorated assets during the period	0
Allowance for credit losses on HTM debt securities at end of period	6
Asset-backed securities | Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of period	$ 5