SEGMENT INFORMATION - Schedule of Industry Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenue from contracts with customers	$ 356,982	$ 384,117
Earnings (Loss) Before Income Tax	19,453	77,143
Segment Assets	2,767,041	2,729,116	$ 2,760,727
Depreciation, Depletion and Amortization	50,724	49,471
Capital Expenditures	34,171	21,956
Coal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	332,502	351,009
Terminal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	17,818	15,221
Freight Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	6,662	17,887
PAMC
Segment Reporting Information [Line Items]
Revenue from contracts with customers	339,164	368,896
Earnings (Loss) Before Income Tax	64,698	98,480
Segment Assets	1,992,549	1,965,979
Depreciation, Depletion and Amortization	44,868	43,257
Capital Expenditures	32,372	19,714
PAMC \| Coal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	332,502	351,009
PAMC \| Terminal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	0	0
PAMC \| Freight Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	6,662	17,887
Other
Segment Reporting Information [Line Items]
Revenue from contracts with customers	17,818	15,221
Earnings (Loss) Before Income Tax	(45,245)	(21,337)
Segment Assets	774,492	763,137
Depreciation, Depletion and Amortization	5,856	6,214
Capital Expenditures	1,799	2,242
Other \| Coal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	0	0
Other \| Terminal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	17,818	15,221
Other \| Freight Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	$ 0	$ 0

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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