REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregation of revenues (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of revenues
Revenues	$ 350,763	$ 526,602
Coal sales
Disaggregation of revenues
Revenues	314,637	476,016
Oil & gas royalties
Disaggregation of revenues
Revenues	14,239	10,393
Transportation revenues
Disaggregation of revenues
Revenues	4,739	30,238
Other revenues
Disaggregation of revenues
Revenues	17,148	9,955
Illinois Basin
Disaggregation of revenues
Revenues	203,872	349,396
Illinois Basin | Coal sales
Disaggregation of revenues
Revenues	199,098	317,270
Illinois Basin | Transportation revenues
Disaggregation of revenues
Revenues	3,856	29,238
Illinois Basin | Other revenues
Disaggregation of revenues
Revenues	918	2,888
Appalachia
Disaggregation of revenues
Revenues	128,103	159,404
Appalachia | Coal sales
Disaggregation of revenues
Revenues	115,539	157,453
Appalachia | Transportation revenues
Disaggregation of revenues
Revenues	883	1,000
Appalachia | Other revenues
Disaggregation of revenues
Revenues	11,681	951
Minerals
Disaggregation of revenues
Revenues	14,263	10,728
Minerals | Oil & gas royalties
Disaggregation of revenues
Revenues	14,239	10,393
Minerals | Other revenues
Disaggregation of revenues
Revenues	24	335
Other and Corporate
Disaggregation of revenues
Revenues	7,384	14,162
Other and Corporate | Coal sales
Disaggregation of revenues
Revenues		5,290
Other and Corporate | Other revenues
Disaggregation of revenues
Revenues	7,384	8,872
Operating segments
Disaggregation of revenues
Revenues	350,763	526,602
Operating segments | Illinois Basin
Disaggregation of revenues
Revenues	203,872	345,399
Operating segments | Appalachia
Disaggregation of revenues
Revenues	128,103	159,404
Operating segments | Minerals
Disaggregation of revenues
Revenues	14,263	10,728
Operating segments | Other and Corporate
Disaggregation of revenues
Revenues	4,525	11,071
Elimination
Disaggregation of revenues
Revenues	(2,859)	(7,088)
Elimination | Coal sales
Disaggregation of revenues
Revenues		(3,997)
Elimination | Other revenues
Disaggregation of revenues
Revenues	(2,859)	(3,091)
Elimination | Illinois Basin
Disaggregation of revenues
Revenues		3,997
Elimination | Other and Corporate
Disaggregation of revenues
Revenues	$ 2,859	$ 3,091