INVESTMENTS - REPURCHASE AGREEMENTS ACCCOUNTED FOR AS BORROWINGS (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Repurchase agreements accounted for as secured borrowings [Line Items]
Securities sold under agreement to repurchase	$ 1,897	$ 1,882
Expense in securities sold under agreement to repurchase	13,446	14,228
Initial Margin
Repurchase agreements accounted for as secured borrowings [Line Items]
Expense in securities sold under agreement to repurchase	7	5
Overnight and Continuous
Repurchase agreements accounted for as secured borrowings [Line Items]
Securities sold under agreement to repurchase	0	0
Up to 30 days
Repurchase agreements accounted for as secured borrowings [Line Items]
Securities sold under agreement to repurchase	1,897	1,882
30–90 days
Repurchase agreements accounted for as secured borrowings [Line Items]
Securities sold under agreement to repurchase	0	0
Greater Than 90 days
Repurchase agreements accounted for as secured borrowings [Line Items]
Securities sold under agreement to repurchase	0	0
U.S. Treasury and agency securities
Repurchase agreements accounted for as secured borrowings [Line Items]
Securities sold under agreement to repurchase	1,897	1,882
U.S. Treasury and agency securities | Overnight and Continuous
Repurchase agreements accounted for as secured borrowings [Line Items]
Securities sold under agreement to repurchase	0	0
U.S. Treasury and agency securities | Up to 30 days
Repurchase agreements accounted for as secured borrowings [Line Items]
Securities sold under agreement to repurchase	1,897	1,882
U.S. Treasury and agency securities | 30–90 days
Repurchase agreements accounted for as secured borrowings [Line Items]
Securities sold under agreement to repurchase	0	0
U.S. Treasury and agency securities | Greater Than 90 days
Repurchase agreements accounted for as secured borrowings [Line Items]
Securities sold under agreement to repurchase	$ 0	$ 0