SEGMENT INFORMATION - Segment Results (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) segment	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Segment information
Number of reportable segments | segment			2
Reportable segment results
Total revenues	$ 551,966	$ 566,342	$ 1,403,732	$ 1,731,261
Segment Adjusted EBITDA Expense	348,932	336,074	848,482	1,045,530
Segment Adjusted EBITDA	196,447	203,652	536,559	612,359
Total assets	2,248,805	2,510,049	2,248,805	2,510,049	$ 2,366,516
Capital expenditures	21,665	51,424	70,267	159,182
Additional information
Equity in income (loss) of affiliates, net	1,105	(17,221)	1,041	(49,049)
Investments in affiliate	128,051		128,051		$ 64,509
Operating segments | Illinois Basin
Reportable segment results
Total revenues	378,488	420,517	962,244	1,237,819
Segment Adjusted EBITDA Expense	246,946	249,615	572,200	728,048
Segment Adjusted EBITDA	126,004	147,522	376,056	445,819
Total assets	1,541,513	1,799,170	1,541,513	1,799,170
Capital expenditures	10,893	37,350	41,264	105,536
Additional information
Equity in income (loss) of affiliates, net		(17,100)		(48,500)
Operating segments | Appalachia
Reportable segment results
Total revenues	154,852	136,741	411,212	460,154
Segment Adjusted EBITDA Expense	95,601	80,421	262,043	296,980
Segment Adjusted EBITDA	57,097	53,380	143,589	154,760
Total assets	480,920	565,213	480,920	565,213
Capital expenditures	10,306	11,616	27,317	49,055
Operating segments | Other and Corporate
Reportable segment results
Total revenues	41,719	40,161	88,253	146,759
Segment Adjusted EBITDA Expense	26,664	33,701	63,575	124,321
Segment Adjusted EBITDA	16,160	6,163	25,555	21,432
Total assets	374,810	282,724	374,810	282,724
Capital expenditures	466	2,458	1,686	4,591
Additional information
Investments in affiliate	128,100	48,000	128,100	48,000
Elimination
Reportable segment results
Total revenues	(23,093)	(31,077)	(57,977)	(113,471)
Segment Adjusted EBITDA Expense	(20,279)	(27,663)	(49,336)	(103,819)
Segment Adjusted EBITDA	(2,814)	(3,413)	(8,641)	(9,652)
Total assets	$ (148,438)	$ (137,058)	$ (148,438)	$ (137,058)