ALLOWANCE FOR CREDIT LOSSES - Schedule of Allowance for Credit Losses for Other Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of year	$ 0
Net credit losses (NCLs)	0
Net reserve builds (releases)	7
Total provision for credit losses	7	$ 0	$ 0
Other, net	22
Allowance for credit losses on other assets at end of year	55	0
Cash and due from banks
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of year	0
Net credit losses (NCLs)	0
Net reserve builds (releases)	(6)
Total provision for credit losses	(6)
Other, net	0
Allowance for credit losses on other assets at end of year	0	0
Deposits with banks
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of year	0
Net credit losses (NCLs)	0
Net reserve builds (releases)	5
Total provision for credit losses	5
Other, net	1
Allowance for credit losses on other assets at end of year	20	0
Securities borrowed or purchased under agreements to resell
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of year	0
Net credit losses (NCLs)	0
Net reserve builds (releases)	8
Total provision for credit losses	8
Other, net	0
Allowance for credit losses on other assets at end of year	10	0
Brokerage receivables
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of year	0
Net credit losses (NCLs)	0
Net reserve builds (releases)	(1)
Total provision for credit losses	(1)
Other, net	0
Allowance for credit losses on other assets at end of year	0	0
All other assets
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of year	0
Net credit losses (NCLs)	0
Net reserve builds (releases)	1
Total provision for credit losses	1
Other, net	21
Allowance for credit losses on other assets at end of year	25	0
Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of year	26
Allowance for credit losses on other assets at end of year		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 year	6
Allowance for credit losses on other assets at end of year		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 year	14
Allowance for credit losses on other assets at end of year		14
Cumulative Effect, Period of Adoption, Adjustment | Securities borrowed or purchased under agreements to resell
Financing Receivable, Other Assets, Allowance For Credit Loss [Roll Forward]
Allowance for credit losses at beginning of year	2
Allowance for credit losses on other assets at end of year		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 year	1
Allowance for credit losses on other assets at end of year		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 year	$ 3
Allowance for credit losses on other assets at end of year		$ 3