SEGMENT INFORMATION - Schedule of Industry Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Coal Revenue	$ 351,009	$ 316,448
Terminal Revenue	15,221	12,886
Freight Revenue	17,887	12,282
Total Revenue and Freight	384,117	341,616
Earnings (Loss) Before Income Tax	77,143	55,851
Segment Assets	2,729,116	2,685,076	$ 2,707,099
Depreciation, Depletion and Amortization	49,471	52,993
Capital Expenditures	21,956	9,021
PAMC
Segment Reporting Information [Line Items]
Coal Revenue	351,009	316,448
Terminal Revenue	0	0
Freight Revenue	17,887	12,282
Total Revenue and Freight	368,896	328,730
Earnings (Loss) Before Income Tax	98,480	61,015
Segment Assets	1,965,979	1,955,172
Depreciation, Depletion and Amortization	43,257	42,301
Capital Expenditures	19,714	8,118
Corporate and Other
Segment Reporting Information [Line Items]
Coal Revenue	0	0
Terminal Revenue	15,221	12,886
Freight Revenue	0	0
Total Revenue and Freight	15,221	12,886
Earnings (Loss) Before Income Tax	(21,337)	(5,164)
Segment Assets	763,137	729,904
Depreciation, Depletion and Amortization	6,214	10,692
Capital Expenditures	2,242	903
Key-Con Fuels | Sales Revenue, Goods, Net | Customer Concentration Risk
Segment Reporting Information [Line Items]
Coal Revenue	86,826
Duke Energy | Sales Revenue, Goods, Net | Customer Concentration Risk
Segment Reporting Information [Line Items]
Coal Revenue	52,213	57,070
Raven Power Fort Smallwood LLC | Sales Revenue, Goods, Net | Customer Concentration Risk
Segment Reporting Information [Line Items]
Coal Revenue	$ 39,262
Xcoal | Sales Revenue, Goods, Net | Customer Concentration Risk
Segment Reporting Information [Line Items]
Coal Revenue		$ 54,820