ACQUISITIONS (Details) $ in Thousands, T in Millions			1 Months Ended		3 Months Ended	5 Months Ended	6 Months Ended
	Feb. 03, 2015 USD ($) T	Dec. 31, 2014 USD ($) T	Feb. 28, 2015 USD ($) T	Feb. 02, 2015 USD ($)	Jun. 30, 2014 $ / T T	Jun. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 $ / T T
Acquisitions
Purchase price paid in cash							$ 28,078
WKY CoalPlay | Acquisition of coal reserves from a subsidiary of Patriot | Coal lease
Acquisitions
Coal reserves leased from related party (in tons) | T	39.1
Payments for royalties			$ 2,100
WKY CoalPlay | Acquisition of coal reserves from a subsidiary of Patriot
Acquisitions
Coal reserves, rights purchased (in tons) | T	39.1		39.1
Purchase price (in dollars)	$ 25,000		$ 25,000
Patriot Coal Corporation
Acquisitions
Purchase price paid in cash	$ 20,500			$ 2,100
Coal reserves (in tons) | T	84.1
Non-reserve coal deposits (in tons) | T	43.2
Increase to purchase price from prior cash paid and agreement for contingent consideration	$ 8,300
Pro forma coal sales under acquired supply agreements (in tons) | T					0.8			1.6
Pro forma average price for coal sales under acquired supply agreements (in dollars per ton) | $ / T					46.67			46.67
Estimated consideration transferred	47,998
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Inventories	3,255
Property, plant and equipment, including mineral rights and leased facilities	30,404
Customer contracts, net	19,193
Other assets	488
Asset retirement obligation	(2,255)
Other liabilities	(3,087)
Net tangible and intangible assets acquired	47,998
Acquisitions, additional information
Agreement for additional payment included in estimated consideration transferred	6,200
Portion of additional agreed consideration amount paid to date						$ 5,300
Patriot Coal Corporation | Preliminary as of March 31, 2015
Acquisitions
Estimated consideration transferred	47,514
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Inventories	3,255
Property, plant and equipment, including mineral rights and leased facilities	26,995
Customer contracts, net	19,193
Other assets	326
Asset retirement obligation	(2,255)
Net tangible and intangible assets acquired	47,514
Patriot Coal Corporation | Adjustments
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Property, plant and equipment, including mineral rights and leased facilities	3,409
Other assets	162
Other liabilities	$ (3,087)
Patriot Coal Corporation | Customer contracts
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
Coal to be delivered under acquired supply agreements (in tons) | T		5.1
Estimated consideration transferred		$ 19,200
Acquisitions, additional information
Amortization expense for acquired coal supply agreements							$ 6,100