EMPLOYEE BENEFIT PLANS - Asset Allocations (Details) - Pension Plan - ARLP	Dec. 31, 2017
Equity securities
Employee Benefit Plans
Target allocation	62.00%
Equity securities | Minimum
Employee Benefit Plans
Target allocation	45.00%
Equity securities | Maximum
Employee Benefit Plans
Target allocation	80.00%
Fixed income securities
Employee Benefit Plans
Target allocation	33.00%
Fixed income securities | Minimum
Employee Benefit Plans
Target allocation	10.00%
Fixed income securities | Maximum
Employee Benefit Plans
Target allocation	55.00%
Real estate
Employee Benefit Plans
Target allocation	5.00%
Real estate | Minimum
Employee Benefit Plans
Target allocation	0.00%
Real estate | Maximum
Employee Benefit Plans
Target allocation	10.00%