ALLOWANCE FOR CREDIT LOSSES - Schedule of Allowance for Credit Losses for HTM Debt Securities (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of period	$ 76,000,000		$ 0
Net credit losses (NCLs)	0		0
Net reserve builds (releases)	31,000,000		37,000,000
Net specific reserve builds (releases)	0		0
Total provision for credit losses on HTM debt securities	31,000,000	$ 0	37,000,000	$ 0
Other, net	0		0
Initial allowance for credit losses on newly purchased credit deteriorated securities during the period	0		0
Allowance for credit losses on HTM debt securities at end of period	107,000,000		107,000,000
State and municipal
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of period	66,000,000		0
Net credit losses (NCLs)	0		0
Net reserve builds (releases)	30,000,000		35,000,000
Net specific reserve builds (releases)	0		0
Total provision for credit losses on HTM debt securities	30,000,000		35,000,000
Other, net	3,000,000		3,000,000
Initial allowance for credit losses on newly purchased credit deteriorated securities during the period	0		0
Allowance for credit losses on HTM debt securities at end of period	99,000,000		99,000,000
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	4,000,000		0
Net credit losses (NCLs)	0		0
Net reserve builds (releases)	2,000,000		2,000,000
Net specific reserve builds (releases)	0		0
Total provision for credit losses on HTM debt securities	2,000,000		2,000,000
Other, net	0		0
Initial allowance for credit losses on newly purchased credit deteriorated securities during the period	0		0
Allowance for credit losses on HTM debt securities at end of period	6,000,000		6,000,000
Asset-based financing
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of period	6,000,000		0
Net credit losses (NCLs)	0		0
Net reserve builds (releases)	(1,000,000)		0
Net specific reserve builds (releases)	0		0
Total provision for credit losses on HTM debt securities	(1,000,000)		0
Other, net	(3,000,000)		(3,000,000)
Initial allowance for credit losses on newly purchased credit deteriorated securities during the period	0		0
Allowance for credit losses on HTM debt securities at end of period	$ 2,000,000		2,000,000
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			70,000,000
Cumulative Effect, Period of Adoption, Adjustment | State and municipal
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of period			61,000,000
Cumulative Effect, Period of Adoption, Adjustment | 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			4,000,000
Cumulative Effect, Period of Adoption, Adjustment | Asset-based financing
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of period			$ 5,000,000