SEGMENT INFORMATION - Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reportable segment results
Total revenues	$ 483,130	$ 453,087	$ 398,619	$ 461,005	$ 527,291	$ 551,966	$ 439,041	$ 412,725	$ 1,795,841	$ 1,931,023	$ 2,273,311
Segment Adjusted EBITDA Expense									1,092,187	1,125,637	1,386,155
Segment Adjusted EBITDA									682,212	778,818	804,513
Total assets	2,221,251				2,197,687				2,221,251	2,197,687	2,366,516
Capital expenditures									145,088	91,056	212,797
Illinois Basin
Reportable segment results
Total revenues									1,059,381	1,275,543	1,527,596
Appalachia
Reportable segment results
Total revenues									623,720	541,108	584,962
Other and Corporate
Reportable segment results
Total revenues									112,740	114,372	160,753
Operating segments | Illinois Basin
Reportable segment results
Total revenues									1,115,478	1,337,160	1,636,217
Segment Adjusted EBITDA Expense									688,468	761,644	961,611
Segment Adjusted EBITDA									391,426	552,284	604,808
Total assets	1,429,078				1,460,924				1,429,078	1,460,924	1,694,044
Capital expenditures									94,252	52,505	145,352
Operating segments | Appalachia
Reportable segment results
Total revenues									626,041	544,914	596,299
Segment Adjusted EBITDA Expense									385,802	346,712	398,071
Segment Adjusted EBITDA									234,124	191,487	186,518
Total assets	470,892				480,745				470,892	480,745	517,972
Capital expenditures									48,358	36,213	61,279
Operating segments | Other and Corporate
Reportable segment results
Total revenues									128,664	132,124	180,622
Segment Adjusted EBITDA Expense									83,490	89,594	153,720
Segment Adjusted EBITDA									65,431	45,909	25,767
Total assets	508,317				408,798				508,317	408,798	269,047
Capital expenditures									2,478	2,338	6,166
Elimination
Reportable segment results
Total revenues									(74,342)	(83,175)	(139,827)
Segment Adjusted EBITDA Expense									(65,573)	(72,313)	(127,247)
Segment Adjusted EBITDA									(8,769)	(10,862)	(12,580)
Total assets	$ (187,036)				$ (152,780)				(187,036)	(152,780)	(114,547)
Elimination | Illinois Basin
Reportable segment results
Total revenues									56,097	61,617	108,621
Elimination | Appalachia
Reportable segment results
Total revenues									2,321	3,806	11,337
Elimination | Other and Corporate
Reportable segment results
Total revenues									$ 15,924	$ 17,752	$ 19,869