SEGMENT INFORMATION - Segment Results (Details) $ in Thousands		3 Months Ended		6 Months Ended
	Apr. 15, 2020 employee	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Reportable segment results
Revenues		$ 255,202	$ 517,054	$ 605,965	$ 1,043,656
Segment Adjusted EBITDA Expense		187,541	319,597	422,239	622,454
Segment Adjusted EBITDA		62,056	165,263	173,757	371,907
Total assets		2,285,628	2,502,721	2,285,628	2,502,721	$ 2,586,694
Capital expenditures		33,881	81,584	84,245	165,627
Gibson County mining complex
Segment Information
Number of employees terminated | employee	116
Hamilton mining complex
Segment Information
Number of employees terminated | employee	78
Illinois Basin
Reportable segment results
Revenues		137,787	335,673	341,659	685,069
Segment Adjusted EBITDA Expense		108,478	208,309	258,465	405,731
Segment Adjusted EBITDA		26,157	96,075	76,186	218,812
Total assets		1,099,463	1,407,019	1,099,463	1,407,019
Capital expenditures		14,029	59,476	40,258	107,930
Appalachia
Reportable segment results
Revenues		107,110	160,244	235,213	319,648
Segment Adjusted EBITDA Expense		73,959	105,122	153,669	204,871
Segment Adjusted EBITDA		30,548	53,779	78,058	112,434
Total assets		501,614	483,265	501,614	483,265
Capital expenditures		19,686	20,987	43,257	54,333
Minerals
Reportable segment results
Revenues		7,847	12,428	22,110	23,156
Segment Adjusted EBITDA Expense		1,119	1,765	2,002	3,592
Segment Adjusted EBITDA		6,881	11,098	20,636	20,230
Total assets		628,308	516,503	628,308	516,503
Other and Corporate
Reportable segment results
Revenues		4,955	15,990	12,339	30,152
Segment Adjusted EBITDA Expense		4,147	9,442	8,789	18,148
Segment Adjusted EBITDA		805	6,551	3,547	24,912
Total assets		462,936	460,353	462,936	460,353
Capital expenditures		166	1,121	730	3,364
Operating segments
Reportable segment results
Revenues		255,202	517,054	605,965	1,043,656
Operating segments | Illinois Basin
Reportable segment results
Revenues		137,787	331,500	341,659	676,899
Operating segments | Appalachia
Reportable segment results
Revenues		107,110	160,244	235,213	319,648
Operating segments | Minerals
Reportable segment results
Revenues		7,847	12,428	22,110	23,156
Operating segments | Other and Corporate
Reportable segment results
Revenues		2,458	12,882	6,983	23,953
Elimination
Reportable segment results
Revenues		(2,497)	(7,281)	(5,356)	(14,369)
Segment Adjusted EBITDA Expense		(162)	(5,041)	(686)	(9,888)
Segment Adjusted EBITDA		(2,335)	(2,240)	(4,670)	(4,481)
Total assets		(406,693)	(364,419)	(406,693)	(364,419)
Elimination | Illinois Basin
Reportable segment results
Revenues			4,173		8,170
Elimination | Other and Corporate
Reportable segment results
Revenues		$ 2,497	$ 3,108	$ 5,356	$ 6,199