Freelandville Purchase (Purchase Price Allocation) (Details) - Freelandville Purchase [Member] $ in Thousands	Mar. 31, 2016 USD ($)
Assets received:
Purchased coal contract	$ 6,407
Advance royalties	1,690
Minerals	9,903
Total	$ 18,000

Amount of assets acquired at the acquisition date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=56956791&loc=d3e4845-128472

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Amount of other assets expected to be realized or consumed before one year or the normal operating cycle, if longer, acquired at the acquisition date.

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Amount of intangible assets, excluding goodwill, acquired at the acquisition date.

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Amount of real estate acquired, at the acquisition date.

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