SEGMENT INFORMATION - Schedule of Industry Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenue from contracts with customers	$ 371,182	$ 404,800	$ 728,165	$ 788,917
Earnings (Loss) Before Income Tax	47,022	55,741	66,475	132,884
Segment Assets	2,765,471	2,756,502	2,765,471	2,756,502	$ 2,760,727
Depreciation, Depletion and Amortization	46,151	54,961	96,875	104,432
Capital Expenditures	48,783	34,243	82,954	56,199
Coal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	350,620	370,697	683,123	721,706
Terminal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	16,708	16,659	34,526	31,880
Freight Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	3,854	17,444	10,516	35,331
PAMC
Segment Reporting Information [Line Items]
Revenue from contracts with customers	354,474	388,141	693,639	757,037
Earnings (Loss) Before Income Tax	60,786	85,173	125,484	182,900
Segment Assets	1,973,627	1,903,324	1,973,627	1,903,324
Depreciation, Depletion and Amortization	45,427	47,582	90,295	90,839
Capital Expenditures	39,813	29,002	72,185	48,717
PAMC \| Coal Revenue
Segment Reporting Information [Line Items]
Revenue from contracts with customers	350,620	370,697	683,123	721,706
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	3,854	17,444	10,516	35,331
Other
Segment Reporting Information [Line Items]
Revenue from contracts with customers	16,708	16,659	34,526	31,880
Earnings (Loss) Before Income Tax	(13,764)	(29,432)	(59,009)	(50,016)
Segment Assets	791,844	853,178	791,844	853,178
Depreciation, Depletion and Amortization	724	7,379	6,580	13,593
Capital Expenditures	8,970	5,241	10,769	7,482
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	16,708	16,659	34,526	31,880
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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