EMPLOYEE BENEFITS - Assumptions Used in Determining Actuarial Present Value of Benefit Obligations and Net Periodic Benefit Cost (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assumptions for benefit obligations
Expected long-term rate of return on plan assets	7.00%	7.50%	7.50%
Assumptions for net periodic pension cost
Expected long-term rate of return on plan assets	7.00%	7.50%	7.50%
Qualified Plan
Assumptions for benefit obligations
Discount rate	3.67%	4.19%	4.64%
Assumptions for net periodic pension cost
Discount rate	4.19%	4.64%	4.125%
Non-Qualified Plan
Assumptions for benefit obligations
Discount rate	3.53%	4.05%	4.54%
Assumptions for net periodic pension cost
Discount rate	4.05%	4.54%	3.875%