ACQUISITIONS - Patriot (Details) $ in Thousands, T in Millions			1 Months Ended	2 Months Ended	12 Months Ended
	Feb. 03, 2015 USD ($) T	Dec. 31, 2014 USD ($) $ / T T	Feb. 28, 2015 USD ($)	Feb. 28, 2015 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Acquisitions, additional information
Agreement for additional payment included in estimated consideration transferred					$ 20,907
Amortization expense for acquired coal supply agreements					15,100	$ 3,000	$ 3,000
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
Coal reserves (in tons) | T	84.1
Non-reserve coal deposits (in tons) | T	43.2
Purchase price paid in cash	$ 20,500			$ 2,100
Increase to purchase price from prior cash paid and agreement for contingent consideration	8,300
Consideration transferred	47,874
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Inventories	1,994
Property, plant and equipment, including mineral rights and leased facilities	32,029
Customer contracts, net	19,193
Asset retirement obligations	(2,255)
Other liabilities	(3,087)
Net tangible and intangible assets acquired	47,874
Acquisitions, additional information
Agreement for additional payment included in estimated consideration transferred	6,200
Patriot Coal Corporation | Preliminary
Acquisitions
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 obligations	(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:
Inventories	(1,261)
Property, plant and equipment, including mineral rights and leased facilities	5,034
Other assets	(326)
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
Purchase price paid in cash		$ 19,200
Coal to be delivered under acquired supply agreements (in tons) | T		5.1
Pro forma coal sales under acquired supply agreements (in tons) | T		3.2
Pro forma average price for coal sales under acquired supply agreements (in dollars per ton) | $ / T		46.67
Revenues generated since the Initial Closing Date					130,500
Acquisitions, additional information
Amortization expense for acquired coal supply agreements					$ 11,800