RELATED-PARTY TRANSACTIONS - Affliliate Royalty Agreements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Related Party Transaction
As of the beginning of period	$ 28,115
As of the end of period	30,579	$ 28,115
Mineral and Coal Leases
Related Party Transaction
As of the beginning of period	16,190	10,706	$ 17,759
Payments	13,819	13,819	3,000
Recoupment	(10,189)	(8,335)	(10,053)
As of the end of period	19,820	16,190	10,706
SGP Land, LLC | MC Mining LLC | Mineral lease
Related Party Transaction
As of the beginning of period	0	0	766
Recoupment			(766)
As of the end of period	0	0	0
SGP | Tunnel Ridge | Coal lease
Related Party Transaction
As of the beginning of period	5,413	10,706	16,993
Payments	3,000	3,000	3,000
Recoupment	(8,413)	(8,293)	(9,287)
As of the end of period	0	5,413	10,706
WKY CoalPlay | December 2014 coal lease - Henderson and Union Counties, Kentucky
Related Party Transaction
As of the beginning of period	3,598	0	0
Payments	3,598	3,598
Recoupment	(1)
As of the end of period	7,195	3,598	0
WKY CoalPlay | December 2014 coal lease - Webster County, Kentucky
Related Party Transaction
As of the beginning of period	2,526	0	0
Payments	2,568	2,568
Recoupment	(1,775)	(42)
As of the end of period	3,319	2,526	0
WKY CoalPlay | December 2014 coal lease - Henderson County, Kentucky
Related Party Transaction
As of the beginning of period	2,522	0	0
Payments	2,522	2,522
As of the end of period	5,044	2,522	0
WKY CoalPlay | February 2015 coal lease - Henderson and Union Counties, Kentucky
Related Party Transaction
As of the beginning of period	2,131	0	0
Payments	2,131	2,131
As of the end of period	$ 4,262	$ 2,131	$ 0