RELATED-PARTY TRANSACTIONS - Affliliate Royalty Agreements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction
As of the beginning of period	$ 30,579
As of the end of period	44,170	$ 30,579
Mineral and Coal Leases
Related Party Transaction
As of the beginning of period	19,820	16,190	$ 10,706
Payments	16,819	13,819	13,819
Recoupment	(3,646)	(10,189)	(8,335)
As of the end of period	32,993	19,820	16,190
SGP | Tunnel Ridge | Coal lease
Related Party Transaction
As of the beginning of period	0	5,413	10,706
Payments	6,000	3,000	3,000
Recoupment	(3,000)	(8,413)	(8,293)
As of the end of period	3,000	0	5,413
WKY CoalPlay | December 2014 coal lease - Henderson and Union Counties, Kentucky
Related Party Transaction
As of the beginning of period	7,195	3,598	0
Payments	3,598	3,598	3,598
Recoupment	(109)	(1)
As of the end of period	10,684	7,195	3,598
WKY CoalPlay | December 2014 coal lease - Webster County, Kentucky
Related Party Transaction
As of the beginning of period	3,319	2,526	0
Payments	2,568	2,568	2,568
Recoupment	(531)	(1,775)	(42)
As of the end of period	5,356	3,319	2,526
WKY CoalPlay | December 2014 coal lease - Henderson County, Kentucky
Related Party Transaction
As of the beginning of period	5,044	2,522	0
Payments	2,522	2,522	2,522
As of the end of period	7,566	5,044	2,522
WKY CoalPlay | February 2015 coal lease - Henderson and Union Counties, Kentucky
Related Party Transaction
As of the beginning of period	4,262	2,131	0
Payments	2,131	2,131	2,131
Recoupment	(6)
As of the end of period	$ 6,387	$ 4,262	$ 2,131