POSTRETIREMENT BENEFIT PLANS - Assumptions Used to Measure Pension Cost (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Principal pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.11%	4.38%	4.02%
Expected return on assets	7.50%	7.50%	7.50%
Other pension plans | Weighted average
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.58%	3.33%	3.53%
Expected return on assets	6.75%	6.36%	6.95%