ACQUISITION OF BUSINESS (Details) (Green River, USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Apr. 02, 2012 T
Business Acquisition
Coal reserves		40,000,000
Consideration paid	$ 100,000
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Inventories	547
Advance royalties	888
Property, plant and equipment, including mineral rights and leased facilities	117,110
Capital lease obligation	(17,384)
Asset retirement obligation	(6,032)
Pneumoconiosis benefits	(2,127)
Net tangible and intangible assets acquired	100,000
Noncompete agreement
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Intangible assets	1,200
Customer contracts, net
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Intangible assets	4,955
Permits
Recognized amounts of net tangible and intangible assets acquired and liabilities assumed:
Intangible assets	$ 843