ALLOWANCE FOR CREDIT LOSSES - Schedule of Allowance for Credit Losses for HTM Debt Securities (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of quarter	$ 78	$ 76	$ 86	$ 0
Gross credit losses	0	0	0	0
Gross recoveries	0	0	3	0
Net credit losses (NCLs)	0	0	3	0
Net reserve builds (releases)	4	31	(4)	37
Net specific reserve builds (releases)	0	0	0	0
Total provision for credit losses on HTM debt securities	4	31	(7)	37
Other, net	1	0	1	0
Initial allowance for credit losses on newly purchased credit-deteriorated securities during the period	0	0	0	0
Allowance for credit losses on HTM debt securities at end of quarter	83	107	83	107
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 quarter	4		3
Gross credit losses	0		0
Gross recoveries	0		3
Net credit losses (NCLs)	0		3
Net reserve builds (releases)	1		2
Net specific reserve builds (releases)	0		0
Total provision for credit losses on HTM debt securities	1		(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 quarter	5		5
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 quarter	69	66	74	0
Gross credit losses	0	0	0	0
Gross recoveries	0	0	0	0
Net credit losses (NCLs)	0	0	0	0
Net reserve builds (releases)	3	30	(2)	35
Net specific reserve builds (releases)	0	0	0	0
Total provision for credit losses on HTM debt securities	3	30	(2)	35
Other, net	0	3	0	3
Initial allowance for credit losses on newly purchased credit-deteriorated securities during the period	0	0	0	0
Allowance for credit losses on HTM debt securities at end of quarter	72	99	72	99
Foreign government
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses on HTM debt securities at beginning of quarter	5	4	6	0
Gross credit losses	0	0	0	0
Gross recoveries	0	0	0	0
Net credit losses (NCLs)	0	0	0	0
Net reserve builds (releases)	0	2	(1)	2
Net specific reserve builds (releases)	0	0	0	0
Total provision for credit losses on HTM debt securities	0	2	(1)	2
Other, net	0	0	0	0
Initial allowance for credit losses on newly purchased credit-deteriorated securities during the period	0	0	0	0
Allowance for credit losses on HTM debt securities at end of quarter	5	6	5	6
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 quarter	0	6	3	0
Gross credit losses	0	0	0	0
Gross recoveries	0	0	0	0
Net credit losses (NCLs)	0	0	0	0
Net reserve builds (releases)	0	(1)	(3)	0
Net specific reserve builds (releases)	0	0	0	0
Total provision for credit losses on HTM debt securities	0	(1)	(3)	0
Other, net	1	(3)	1	(3)
Initial allowance for credit losses on newly purchased credit-deteriorated securities during the period	0	0	0	0
Allowance for credit losses on HTM debt securities at end of quarter	$ 1	2	$ 1	2
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 quarter		0		70
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 quarter		0		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 quarter		0		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 quarter		$ 0		$ 5