SEGMENT INFORMATION - Schedule of Industry Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenue from contracts with customers	$ 404,800	$ 336,324	$ 788,917	$ 677,942
Earnings (Loss) Before Income Tax	55,741	61,857	132,884	117,707
Segment Assets	2,756,502	2,626,610	2,756,502	2,626,610	$ 2,707,099
Depreciation, Depletion and Amortization	54,961	25,268	104,432	78,261
Capital Expenditures	34,243	14,208	56,199	23,229
Coal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	370,697	303,707	721,706	620,155
Terminal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	16,659	14,855	31,880	27,742
Freight Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	17,444	17,762	35,331	30,045
PAMC
Segment Reporting Information [Line Items]
Revenue from contracts with customers	388,141	321,469	757,037	650,200
Earnings (Loss) Before Income Tax	85,173	49,643	182,900	110,658
Segment Assets	1,903,324	1,938,811	1,903,324	1,938,811
Depreciation, Depletion and Amortization	47,582	41,402	90,839	83,703
Capital Expenditures	29,002	13,770	48,717	21,888
PAMC \| Coal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	370,697	303,707	721,706	620,155
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	17,444	17,762	35,331	30,045
Other
Segment Reporting Information [Line Items]
Revenue from contracts with customers	16,659	14,855	31,880	27,742
Earnings (Loss) Before Income Tax	(29,432)	12,214	(50,016)	7,049
Segment Assets	853,178	687,799	853,178	687,799
Depreciation, Depletion and Amortization	7,379	(16,134)	13,593	(5,442)
Capital Expenditures	5,241	438	7,482	1,341
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	16,659	14,855	31,880	27,742
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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