PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS - Fair Value and Aggregate Notional Amount of the Derivative Instruments Held by our Pension Trusts (Details) - Pension - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Derivatives, fair value, net	$ 277	$ 476
Derivative, notional amount	1,644	3,357
Equity and fixed income index derivatives, net
Defined Benefit Plan Disclosure [Line Items]
Derivatives, fair value, net	0	19
Derivative, notional amount	0	501
Foreign currency derivatives, net
Defined Benefit Plan Disclosure [Line Items]
Derivatives, fair value, net	0	12
Derivative, notional amount	13	1,413
Future [Member]
Defined Benefit Plan Disclosure [Line Items]
Derivatives, fair value, net	15	0
Derivative, notional amount	1,073	0
Total return swaps, net
Defined Benefit Plan Disclosure [Line Items]
Derivatives, fair value, net	262	445
Derivative, notional amount	$ 558	$ 1,443