ALLOWANCE FOR CREDIT LOSSES - Schedule of Allowance for Credit Losses for HTM Debt Securities (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 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	$ 83	$ 107	$ 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)	(6)	(16)	(10)	21
Net specific reserve builds (releases)	(4)	0	(4)	0
Total provision for credit losses on HTM debt securities	(10)	(16)	(17)	21
Other, net	0	7	1	7
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	73	98	73	98
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	5	0	3	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)	0	3	2	3
Net specific reserve builds (releases)	(4)	0	(4)	0
Total provision for credit losses on HTM debt securities	(4)	3	(5)	3
Other, net	0	5	0	5
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	8	1	8
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	72	99	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)	(5)	(19)	(7)	16
Net specific reserve builds (releases)	0	0	0	0
Total provision for credit losses on HTM debt securities	(5)	(19)	(7)	16
Other, net	0	0	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	67	80	67	80
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	6	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)	(1)	(1)	(2)	1
Net specific reserve builds (releases)	0	0	0	0
Total provision for credit losses on HTM debt securities	(1)	(1)	(2)	1
Other, net	0	2	0	2
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	4	7	4	7
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	1	2	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)	1
Net specific reserve builds (releases)	0	0	0	0
Total provision for credit losses on HTM debt securities	0	1	(3)	1
Other, net	0	0	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	3	$ 1	3
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 | 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		0		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 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