PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS - Estimated Amortization from Cumulative Other Comprehensive Loss (Details) $ in Millions	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 250
Prior service cost (credit)	(5)
Net effect cost	245
Pension
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	242
Prior service cost (credit)	3
Net effect cost	245
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	8
Prior service cost (credit)	(8)
Net effect cost	$ 0