POSTRETIREMENT BENEFIT PLANS - Assumptions Used to Measure Pension Benefit Obligations (Details)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Principal pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.64%	4.11%	4.38%
Compensation increases	3.55%	3.80%	3.80%
Other pension plans | Weighted average
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.45%	2.58%	3.33%
Compensation increases	3.12%	3.48%	3.32%