SEGMENT INFORMATION - Schedule of Industry Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenue from contracts with customers	$ 313,355	$ 316,113	$ 1,102,272	$ 994,053
Earnings (Loss) Before Income Tax	8,394	12,288	141,278	129,996
Segment Assets	2,745,928	2,588,529	2,745,928	2,588,529	$ 2,707,099
Depreciation, Depletion and Amortization	51,242	46,653	155,674	124,914
Capital Expenditures	40,656	27,781	96,855	51,010
Coal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	294,797	279,245	1,016,503	899,400
Terminal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	16,115	15,065	47,995	42,806
Freight Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	2,443	21,803	37,774	51,847
PAMC
Segment Reporting Information [Line Items]
Revenue from contracts with customers	297,240	301,048	1,054,277	951,247
Earnings (Loss) Before Income Tax	37,962	21,011	220,862	131,670
Segment Assets	1,891,606	1,912,656	1,891,606	1,912,656
Depreciation, Depletion and Amortization	44,236	41,638	135,074	125,341
Capital Expenditures	32,309	27,157	81,025	49,045
PAMC \| Coal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	294,797	279,245	1,016,503	899,400
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	2,443	21,803	37,774	51,847
Other
Segment Reporting Information [Line Items]
Revenue from contracts with customers	16,115	15,065	47,995	42,806
Earnings (Loss) Before Income Tax	(29,568)	(8,723)	(79,584)	(1,674)
Segment Assets	854,322	675,873	854,322	675,873
Depreciation, Depletion and Amortization	7,006	5,015	20,600	(427)
Capital Expenditures	8,347	624	15,830	1,965
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	16,115	15,065	47,995	42,806
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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The cash outflow for purchases of and capital improvements on property, plant and equipment (capital expenditures), software, and other intangible assets.

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