MISCELLANEOUS OTHER INCOME (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Other Income and Expenses [Abstract]
Royalty Income - Non-Operated Coal	$ 5,160	$ 3,520	$ 19,108	$ 15,713
Purchased Coal Sales	2,901	3,569	15,389	9,667
Property Easements and Option Income	1,069	1,402	5,479	2,396
Rental Income	896	1,589	3,066	12,722
Interest Income	523	448	1,591	1,495
Contract Buyout	0	8,410	0	8,410
Other	429	775	2,601	2,105
Miscellaneous Other Income	$ 10,978	$ 19,713	$ 47,234	$ 52,508