EMPLOYEE BENEFIT PLANS - Estimated Benefit Payments (Details) - ARLP - Pension Plan $ in Thousands	Dec. 31, 2017 USD ($)
Estimated future benefit payments
2018	$ 4,238
2019	4,651
2020	5,053
2021	5,420
2022	5,696
2023-2027	32,329
Estimated future benefit payments	57,387
Expected contribution for pension plan in 2018	3,800
Estimated net actuarial loss for pension plan amortized from AOCI into net periodic benefit cost in 2018	3,800
Estimated prior service cost for pension plan amortized from AOCI into net periodic benefit cost in 2018	$ 200