SEGMENT INFORMATION - Schedule of Industry Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenue from contracts with customers	$ 321,444	$ 313,355	$ 1,049,609	$ 1,102,272
Earnings (Loss) Before Income Tax	9,439	8,394	75,914	141,278
Segment Assets	2,723,235	2,745,928	2,723,235	2,745,928	$ 2,760,727
Depreciation, Depletion and Amortization	54,370	51,242	151,245	155,674
Capital Expenditures	48,521	40,656	131,475	96,855
Coal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	301,542	294,797	984,665	1,016,503
Terminal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	16,303	16,115	50,829	47,995
Freight Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	3,599	2,443	14,115	37,774
PAMC
Segment Reporting Information [Line Items]
Revenue from contracts with customers	305,141	297,240	998,780	1,054,277
Earnings (Loss) Before Income Tax	30,546	37,962	156,029	220,862
Segment Assets	2,014,381	1,891,606	2,014,381	1,891,606
Depreciation, Depletion and Amortization	45,829	44,236	136,124	135,074
Capital Expenditures	45,232	32,309	117,417	81,025
PAMC \| Coal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	301,542	294,797	984,665	1,016,503
PAMC \| Terminal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	0	0	0	0
PAMC \| Freight Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	3,599	2,443	14,115	37,774
Other
Segment Reporting Information [Line Items]
Revenue from contracts with customers	16,303	16,115	50,829	47,995
Earnings (Loss) Before Income Tax	(21,107)	(29,568)	(80,115)	(79,584)
Segment Assets	708,854	854,322	708,854	854,322
Depreciation, Depletion and Amortization	8,541	7,006	15,121	20,600
Capital Expenditures	3,289	8,347	14,058	15,830
Other \| Coal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	0	0	0	0
Other \| Terminal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	16,303	16,115	50,829	47,995
Other \| Freight Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	$ 0	$ 0	$ 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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