ALLOWANCE FOR CREDIT LOSSES - Schedule of Allowance for Credit Losses for Other Assets (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	$ 55	$ 0
Net credit losses (NCLs)	0	0
Net reserve builds (releases)	9	(4)
Total provision for credit losses	9	(4)
Other, net	(1)	32
Allowance for credit losses at end of period	63	54
Cash and due from banks
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	0	0
Net credit losses (NCLs)	0	0
Net reserve builds (releases)	0	(6)
Total provision for credit losses	0	(6)
Other, net	0	0
Allowance for credit losses at end of period	0	0
Deposits with banks
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	20	0
Net credit losses (NCLs)	0	0
Net reserve builds (releases)	9	(6)
Total provision for credit losses	9	(6)
Other, net	(1)	0
Allowance for credit losses at end of period	28	8
Securities borrowed and purchased under agreements to resell
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	10	0
Net credit losses (NCLs)	0	0
Net reserve builds (releases)	(5)	3
Total provision for credit losses	(5)	3
Other, net	0	0
Allowance for credit losses at end of period	5	5
Brokerage receivables
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	0	0
Net credit losses (NCLs)	0	0
Net reserve builds (releases)	0	(1)
Total provision for credit losses	0	(1)
Other, net	0	0
Allowance for credit losses at end of period	0	0
All other assets
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period	25	0
Net credit losses (NCLs)	0	0
Net reserve builds (releases)	5	6
Total provision for credit losses	5	6
Other, net	0	32
Allowance for credit losses at end of period	$ 30	41
Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period		26
Cumulative Effect, Period of Adoption, Adjustment | Cash and due from banks
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period		6
Cumulative Effect, Period of Adoption, Adjustment | Deposits with banks
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period		14
Cumulative Effect, Period of Adoption, Adjustment | Securities borrowed and purchased under agreements to resell
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period		2
Cumulative Effect, Period of Adoption, Adjustment | Brokerage receivables
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period		1
Cumulative Effect, Period of Adoption, Adjustment | All other assets
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of period		$ 3