SEGMENT INFORMATION - Schedule of Industry Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Coal Sales	$ 279,245	$ 267,685	$ 899,400	$ 744,411
Other Outside Sales	15,065	4,549	42,806	20,316
Freight Revenue	21,803	9,392	51,847	33,949
Total Sales and Freight	316,113	281,626	994,053	798,676
Earnings (Loss) Before Income Tax	12,288	4,685	129,996	27,277
Segment Assets	2,588,529	2,769,557	2,588,529	2,769,557	$ 2,687,434
Depreciation, Depletion and Amortization	46,653	49,850	124,914	127,826
Capital Expenditures	27,781	15,455	51,010	42,661
PAMC
Segment Reporting Information [Line Items]
Coal Sales	279,245	267,685	899,400	744,411
Other Outside Sales	0	0	0	0
Freight Revenue	21,803	9,392	51,847	33,949
Total Sales and Freight	301,048	277,077	951,247	778,360
Earnings (Loss) Before Income Tax	21,011	34,741	131,670	80,588
Segment Assets	1,912,656	2,007,767	1,912,656	2,007,767
Depreciation, Depletion and Amortization	41,638	42,370	125,341	125,334
Capital Expenditures	27,157	12,292	49,045	38,295
Corporate and Other
Segment Reporting Information [Line Items]
Coal Sales	0	0	0	0
Other Outside Sales	15,065	4,549	42,806	20,316
Freight Revenue	0	0	0	0
Total Sales and Freight	15,065	4,549	42,806	20,316
Earnings (Loss) Before Income Tax	(8,723)	(30,056)	(1,674)	(53,311)
Segment Assets	675,873	761,790	675,873	761,790
Depreciation, Depletion and Amortization	5,015	7,480	(427)	2,492
Capital Expenditures	624	3,163	1,965	4,366
Duke Energy | Sales Revenue, Goods, Net | Customer Concentration Risk
Segment Reporting Information [Line Items]
Coal Sales	$ 70,159	47,939	177,948	$ 107,618
GenOn Energy Management, LLC | Sales Revenue, Goods, Net | Customer Concentration Risk
Segment Reporting Information [Line Items]
Coal Sales		$ 32,431
Xcoal Energy Resources | Sales Revenue, Goods, Net | Customer Concentration Risk
Segment Reporting Information [Line Items]
Coal Sales			$ 114,451