MISCELLANEOUS OTHER INCOME (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other Income and Expenses [Abstract]
Royalty Income - Non-Operated Coal	$ 4,142	$ 3,496	$ 13,948	$ 12,193
Purchased Coal Sales	3,742	2,557	12,488	6,098
Rental Income	1,048	2,606	2,170	11,133
Interest Income	467	457	1,068	1,047
Property Easements and Option Income	259	602	4,410	994
Other	711	427	2,172	1,329
Miscellaneous Other Income	$ 10,369	$ 10,145	$ 36,256	$ 32,794