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, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Derivatives, fair value, net	$ 476	$ 378
Derivative, notional amount	3,357	4,731
Equity and fixed income index derivatives, net
Defined Benefit Plan Disclosure [Line Items]
Derivatives, fair value, net	19	10
Derivative, notional amount	501	405
Foreign currency derivatives, net
Defined Benefit Plan Disclosure [Line Items]
Derivatives, fair value, net	12	(5)
Derivative, notional amount	1,413	2,811
Total return swaps, net
Defined Benefit Plan Disclosure [Line Items]
Derivatives, fair value, net	445	373
Derivative, notional amount	$ 1,443	$ 1,515