SEGMENT INFORMATION - EBITDA Expense Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reconciliation of consolidated Segment Adjusted EBITDA Expense to operating expenses (excluding depreciation, depletion and amortization)
Segment Adjusted EBITDA Expense	$ 294,611	$ 328,112	$ 794,384	$ 843,386
Outside coal purchases		(1,514)		(1,514)
Other income	774	293	2,461	545
Operating expenses (excluding depreciation, depletion and amortization)	$ 295,385	$ 326,891	$ 796,845	$ 842,417