SEGMENT INFORMATION - EBITDA Expense Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Reconciliation of consolidated Segment Adjusted EBITDA Expense to operating expenses (excluding depreciation, depletion and amortization)
Segment Adjusted EBITDA Expense	$ 348,932	$ 336,074	$ 848,482	$ 1,045,530
Outside coal purchases	(1,514)	(2)	(1,514)	(326)
Other income	293	455	545	750
Operating expenses (excluding depreciation, depletion and amortization)	$ 347,711	$ 336,527	$ 847,513	$ 1,045,954