SEGMENT INFORMATION - EBITDA Expense Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Reconciliation of consolidated Segment Adjusted EBITDA Expense to operating expenses (excluding depreciation, depletion and amortization)
Segment Adjusted EBITDA Expense	$ 187,541	$ 319,597	$ 422,239	$ 622,454
Outside coal purchases		(5,311)		(5,311)
Other expense	(377)	(13)	(733)	(142)
Operating expenses (excluding depreciation, depletion and amortization)	$ 187,164	$ 314,273	$ 421,506	$ 617,001