COMMITMENT AND CONTINGENT LIABILITIES - Obligations Under Funding Agreements (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Restructuring Cost and Reserve [Line Items]
Unaccrued amounts of reasonably possible range of losses	$ 100
Advances from Federal Home Loan Banks	3,990	$ 3,990
Issued During the Period	5,940
Repaid During the Period	5,940
Long-term agreements maturing within one year	0
Funding Agreement, Difference Related To Remaining Amortization	11	12
Federal Home Loan Bank of New York Short-Term Funding Agreements Maturing in Less than One Month
Restructuring Cost and Reserve [Line Items]
Advances from Federal Home Loan Banks	1,698	1,640
Issued During the Period	5,940
Repaid During the Period	5,940
Long-term agreements maturing within one year	58
Federal Home Loan Bank of New York Long-Term Funding Agreements Maturing in Less than Five Years
Restructuring Cost and Reserve [Line Items]
Advances from Federal Home Loan Banks	1,511	1,569
Issued During the Period	0
Repaid During the Period	0
Long-term agreements maturing within one year	(58)
Federal Home Loan Bank of New York Long-Term Funding Agreements Maturing in Greater than Five Years
Restructuring Cost and Reserve [Line Items]
Advances from Federal Home Loan Banks	781	781
Issued During the Period	0
Repaid During the Period	0
Federal Home Loan Bank of New York Long-Term Funding Agreements
Restructuring Cost and Reserve [Line Items]
Advances from Federal Home Loan Banks	2,292	$ 2,350
Issued During the Period	0
Repaid During the Period	0
Long-term agreements maturing within one year	$ (58)