EMPLOYEE BENEFIT PLANS - Assumptions (Details) - Pension Plan - ARLP	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Asset allocation assumptions
Active management premium percentage	1.50%
Asset allocation assumption	100.00%
Actual return on plan assets	18.00%	5.90%
Equity securities
Asset allocation assumptions
Asset allocation assumption	62.00%
Fixed income securities
Asset allocation assumptions
Asset allocation assumption	33.00%
Real estate
Asset allocation assumptions
Asset allocation assumption	5.00%