EMPLOYEE BENEFIT PLANS - Weighted-Average Assumptions Used to Determine Benefit Obligations and Net Periodic Benefit Cost (Details) - Qualified Pension Plan	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted-average assumptions used to determine net periodic benefit cost
Annualized returns obtained period	25 years
Qualified Plan
Weighted-average assumptions used to determine benefit obligations
Discount rate (as a percent)	3.55%	4.04%
Rate of compensation increase prior to age 40 (as a percent)	4.75%	4.75%
Rate of compensation increase age 40 and above (as a percent)	3.75%	3.75%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	4.04%	4.29%	3.95%
Rate of compensation increase prior to age 40 (as a percent)	4.75%	4.75%	4.75%
Rate of compensation increase age 40 and above (as a percent)	3.75%	3.75%	3.75%
Expected long-term return on plan assets (as a percent)	7.00%	7.25%	7.50%
Nonqualified Plan
Weighted-average assumptions used to determine benefit obligations
Discount rate (as a percent)	3.26%	3.60%
Rate of compensation increase prior to age 40 (as a percent)	4.75%	4.75%
Rate of compensation increase age 40 and above (as a percent)	3.75%	3.75%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	3.60%	3.63%	3.65%
Rate of compensation increase prior to age 40 (as a percent)	4.75%	4.75%	4.75%
Rate of compensation increase age 40 and above (as a percent)	3.75%	3.75%	3.75%