SEGMENT INFORMATION - Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Reportable segment results
Total revenues	$ 398,619	$ 439,041	$ 859,624	$ 851,766
Segment Adjusted EBITDA Expense	238,279	251,786	499,773	515,274
Segment Adjusted EBITDA	155,928	181,736	349,543	324,388
Total assets	2,181,799	2,372,735	2,181,799	2,372,735	$ 2,197,687
Capital expenditures	37,171	16,869	67,517	48,602
Additional information
Equity in income (loss) of affiliates	2,916	(37)	6,616	(64)
Investments in affiliate	149,592		149,592		$ 138,817
Illinois Basin
Reportable segment results
Total revenues	232,484	285,557	496,834	561,575
Appalachia
Reportable segment results
Total revenues	135,706	136,409	305,950	252,554
Other and Corporate
Reportable segment results
Total revenues	30,429	17,075	56,840	37,637
Operating segments | Illinois Basin
Reportable segment results
Total revenues	249,573	295,880	525,128	583,756
Segment Adjusted EBITDA Expense	150,299	161,045	311,736	339,688
Segment Adjusted EBITDA	93,288	130,846	199,551	235,618
Total assets	1,445,624	1,658,518	1,445,624	1,658,518
Capital expenditures	22,927	12,357	40,116	30,371
Operating segments | Appalachia
Reportable segment results
Total revenues	135,706	138,034	305,950	256,360
Segment Adjusted EBITDA Expense	83,619	88,003	178,936	167,732
Segment Adjusted EBITDA	50,744	48,532	123,931	85,202
Total assets	473,699	516,584	473,699	516,584
Capital expenditures	13,261	4,348	26,041	17,011
Operating segments | Other and Corporate
Reportable segment results
Total revenues	34,324	21,468	64,848	46,534
Segment Adjusted EBITDA Expense	23,123	16,165	40,961	36,911
Segment Adjusted EBITDA	14,117	5,271	30,503	9,395
Total assets	373,566	363,448	373,566	363,448
Capital expenditures	983	164	1,360	1,220
Additional information
Equity in income (loss) of affiliates	2,900		6,600
Investments in affiliate	149,600	96,700	149,600	96,700
Elimination
Reportable segment results
Total revenues	(20,984)	(16,341)	(36,302)	(34,884)
Segment Adjusted EBITDA Expense	(18,762)	(13,427)	(31,860)	(29,057)
Segment Adjusted EBITDA	(2,221)	(2,913)	(4,442)	(5,827)
Total assets	(111,090)	(165,815)	(111,090)	(165,815)
Elimination | Illinois Basin
Reportable segment results
Total revenues	17,089	10,323	28,294	22,181
Elimination | Appalachia
Reportable segment results
Total revenues		1,625		3,806
Elimination | Other and Corporate
Reportable segment results
Total revenues	$ 3,895	$ 4,393	$ 8,008	$ 8,897