COMMITMENTS AND CONTINGENT LIABILITIES - Funding Agreements (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Outstanding balance at end of period	$ 6,750	$ 6,900
Issued during the period	11,750
Repaid during the period	11,900
Long-term Agreements Maturing Within One Year	0
Difference related to remaining amortization	9	9
Federal Home Loan Bank of New York Short-Term Funding Agreements Maturing in Less than One Year
Debt Instrument [Line Items]
Outstanding balance at end of period	4,632	4,608
Issued during the period	11,750
Repaid during the period	11,900
Long-term Agreements Maturing Within One Year	174
Federal Home Loan Bank of New York Long-Term Funding Agreements Maturing Between Two and Five Years
Debt Instrument [Line Items]
Outstanding balance at end of period	1,472	1,646
Issued during the period	0
Repaid during the period	0
Long-term Agreements Maturing Within One Year	(174)
Federal Home Loan Bank of New York Long-Term Funding Agreements Maturing in Greater than Five Years
Debt Instrument [Line Items]
Outstanding balance at end of period	646	646
Issued during the period	0
Repaid during the period	0
Long-term Agreements Maturing Within One Year	0
Federal Home Loan Bank of New York Long-Term Funding Agreements
Debt Instrument [Line Items]
Outstanding balance at end of period	2,119	$ 2,292
Issued during the period	0
Repaid during the period	0
Long-term Agreements Maturing Within One Year	$ (174)