ACQUISITIONS - Patriot (Details) $ in Thousands, T in Millions			1 Months Ended	2 Months Ended	12 Months Ended
	Feb. 03, 2015 USD ($) T	Dec. 31, 2014 USD ($) property T	Feb. 28, 2015 USD ($)	Feb. 28, 2015 USD ($)	Dec. 31, 2015 USD ($)
Acquisitions
Liabilities assumed in acquisition					$ 20,907
Coal Reserves in Henderson and Union Counties, Kentucky | WKY CoalPlay | Coal lease | Variable Interest Entity, Not Primary Beneficiary
Acquisitions
Coal reserves leased from related party (in tons) | T	39.1
WKY CoalPlay | Coal Reserves in Henderson and Union Counties, Kentucky | Central States, a subsidiary of Patriot
Acquisitions
Coal reserves, rights purchased (in tons) | T	39.1
Purchase price	$ 25,000
Patriot Coal Corporation
Acquisitions
Number of mining operations | property		2
Consideration transferred	$ 47,874
Coal reserves (in tons) | T	84.1
Non-reserve coal deposits (in tons) | T	43.2
Purchase price paid in cash	$ 20,500	$ 19,200		$ 2,100
Increase to purchase price from prior cash paid and agreement for contingent consideration	8,300
Liabilities assumed in acquisition	$ 2,300	6,200
Patriot Coal Corporation | Customer contracts and other, net
Acquisitions
Initial purchase price for coal supply agreements		21,000
Amount paid into escrow		$ 9,300
Escrow deposit released			$ 7,500
Escrow deposit returned			1,800
Consideration transferred			$ 19,200
Coal to be delivered under acquired supply agreements (in tons) | T		5.1
Revenues generated since the Initial Closing Date					$ 130,500