ALLOWANCE FOR CREDIT LOSSES - Schedule of Allowance for Credit Losses for HTM Debt Securities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of period	$ 86	$ 0
Gross credit losses	0
Gross recoveries	3
Net credit losses (NCLs)	3	0
Net reserve builds (releases)	(8)	6
Net specific reserve builds (releases)	0	0
Total provision for credit losses on HTM debt securities	(11)	6
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	78	76
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	3	0
Gross credit losses	0
Gross recoveries	3
Net credit losses (NCLs)	3	0
Net reserve builds (releases)	1	0
Net specific reserve builds (releases)	0	0
Total provision for credit losses on HTM debt securities	(2)	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	4	0
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	74	0
Gross credit losses	0
Gross recoveries	0
Net credit losses (NCLs)	0	0
Net reserve builds (releases)	(5)	5
Net specific reserve builds (releases)	0	0
Total provision for credit losses on HTM debt securities	(5)	5
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	69	66
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	0
Gross credit losses	0
Gross recoveries	0
Net credit losses (NCLs)	0	0
Net reserve builds (releases)	(1)	0
Net specific reserve builds (releases)	0	0
Total provision for credit losses on HTM debt securities	(1)	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	5	4
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	3	0
Gross credit losses	0
Gross recoveries	0
Net credit losses (NCLs)	0	0
Net reserve builds (releases)	(3)	1
Net specific reserve builds (releases)	0	0
Total provision for credit losses on HTM debt securities	(3)	1
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	$ 0	6
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
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
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
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