REVENUE (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Total Revenue from Contracts with Customers	$ 404,800	$ 336,324	$ 788,917	$ 677,942
Coal Revenue
Disaggregation of Revenue [Line Items]
Total Revenue from Contracts with Customers	370,697	303,707	721,706	620,155
Terminal Revenue
Disaggregation of Revenue [Line Items]
Total Revenue from Contracts with Customers	16,659	14,855	31,880	27,742
Freight Revenue
Disaggregation of Revenue [Line Items]
Total Revenue from Contracts with Customers	$ 17,444	$ 17,762	$ 35,331	$ 30,045