ALLOWANCE FOR CREDIT LOSSES - Schedule of Allowance for Credit Losses for HTM Debt Securities (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 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	$ 107,000,000		$ 0
Net credit losses (NCLs)	0		0
Net reserve builds (releases)	(16,000,000)		21,000,000
Net specific reserve builds (releases)	0		0
Total provision for credit losses on HTM debt securities	(16,000,000)	$ 0	21,000,000	$ 0
Other, net	7,000,000		7,000,000
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	98,000,000		98,000,000
Mortgage-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		0
Net credit losses (NCLs)	0		0
Net reserve builds (releases)	3,000,000		3,000,000
Net specific reserve builds (releases)	0		0
Total provision for credit losses on HTM debt securities	3,000,000		3,000,000
Other, net	5,000,000		5,000,000
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	8,000,000		8,000,000
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	99,000,000		0
Net credit losses (NCLs)	0		0
Net reserve builds (releases)	(19,000,000)		16,000,000
Net specific reserve builds (releases)	0		0
Total provision for credit losses on HTM debt securities	(19,000,000)		16,000,000
Other, net	0		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	80,000,000		80,000,000
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	6,000,000		0
Net credit losses (NCLs)	0		0
Net reserve builds (releases)	(1,000,000)		1,000,000
Net specific reserve builds (releases)	0		0
Total provision for credit losses on HTM debt securities	(1,000,000)		1,000,000
Other, net	2,000,000		2,000,000
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	7,000,000		7,000,000
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	2,000,000		0
Net credit losses (NCLs)	0		0
Net reserve builds (releases)	1,000,000		1,000,000
Net specific reserve builds (releases)	0		0
Total provision for credit losses on HTM debt securities	1,000,000		1,000,000
Other, net	0		(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	$ 3,000,000		3,000,000
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 | Mortgage-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
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