EMPLOYEE BENEFIT PLANS - Assumptions (Details) - Pension Plan - ARLP	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Asset allocation assumptions
Active management premium percentage	1.50%
Asset allocation assumption	100.00%
Expected rate of return on plan assets	7.00%	7.50%
Actual return on plan assets	5.90%	(2.00%)
Domestic Equity Securities
Asset allocation assumptions
Asset allocation assumption	70.00%
Expected rate of return on plan assets	6.40%
Foreign equity securities
Asset allocation assumptions
Asset allocation assumption	10.00%
Expected rate of return on plan assets	6.80%
Fixed income securities/cash
Asset allocation assumptions
Asset allocation assumption	20.00%
Expected rate of return on plan assets	3.00%