SEGMENT INFORMATION - 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
Reportable segment results
Total revenues	$ 453,087	$ 551,966	$ 1,312,711	$ 1,403,732
Segment Adjusted EBITDA Expense	294,611	328,112	794,384	843,386
Segment Adjusted EBITDA	157,065	217,267	506,608	541,655
Total assets	2,210,759	2,248,805	2,210,759	2,248,805	$ 2,197,687
Capital expenditures	37,938	21,665	105,455	70,267
Additional information
Equity in income (loss) of affiliates	3,798	1,105	10,414	1,041
Investments in affiliate	144,349		144,349		$ 138,817
Illinois Basin
Reportable segment results
Total revenues	266,931	360,221	763,765	921,796
Appalachia
Reportable segment results
Total revenues	152,865	154,852	458,815	407,406
Other and Corporate
Reportable segment results
Total revenues	33,291	36,893	90,131	74,530
Operating segments | Illinois Basin
Reportable segment results
Total revenues	285,377	378,488	810,505	962,244
Segment Adjusted EBITDA Expense	192,465	227,526	504,201	567,214
Segment Adjusted EBITDA	86,377	145,424	285,928	381,042
Total assets	1,406,831	1,541,513	1,406,831	1,541,513
Capital expenditures	26,328	10,893	66,444	41,264
Operating segments | Appalachia
Reportable segment results
Total revenues	154,255	154,852	460,205	411,212
Segment Adjusted EBITDA Expense	97,316	94,201	276,252	261,933
Segment Adjusted EBITDA	55,465	58,497	179,396	143,699
Total assets	468,580	480,920	468,580	480,920
Capital expenditures	11,126	10,306	37,167	27,317
Operating segments | Other and Corporate
Reportable segment results
Total revenues	37,268	41,719	102,116	88,253
Segment Adjusted EBITDA Expense	26,421	26,664	67,382	63,575
Segment Adjusted EBITDA	17,445	16,160	47,948	25,555
Total assets	458,421	374,810	458,421	374,810
Capital expenditures	484	466	1,844	1,686
Elimination
Reportable segment results
Total revenues	(23,813)	(23,093)	(60,115)	(57,977)
Segment Adjusted EBITDA Expense	(21,591)	(20,279)	(53,451)	(49,336)
Segment Adjusted EBITDA	(2,222)	(2,814)	(6,664)	(8,641)
Total assets	(123,073)	(148,438)	(123,073)	(148,438)
Elimination | Illinois Basin
Reportable segment results
Total revenues	18,446	18,267	46,740	40,448
Elimination | Appalachia
Reportable segment results
Total revenues	1,390		1,390	3,806
Elimination | Other and Corporate
Reportable segment results
Total revenues	$ 3,977	$ 4,826	$ 11,985	$ 13,723