SEGMENT INFORMATION - EBITDA Expense Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of consolidated Segment Adjusted EBITDA Expense to operating expenses (excluding depreciation, depletion and amortization)
Segment Adjusted EBITDA Expense	$ 1,092,187	$ 1,125,637	$ 1,386,155
Outside coal purchases		(1,514)	(327)
Other income	2,980	725	955
Operating expenses (excluding depreciation, depletion and amortization)	$ 1,095,167	$ 1,124,848	$ 1,386,783