Asset Retirement Obligations (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Oct. 01, 2010
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of period	$ 32,415,000us-gaap_AssetRetirementObligation	$ 31,486,000us-gaap_AssetRetirementObligation
Liabilities acquired during the period	7,297,000
Liabilities settled during the period	(1,509,000)	(1,559,000)
Accretion expense	1,562,000	1,896,000
Revision of estimated cash flows	2,054,000	592,000
Balance at end of period	41,819,000us-gaap_AssetRetirementObligation	32,415,000us-gaap_AssetRetirementObligation
NACoal
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of period	15,489,000	15,070,000
Liabilities acquired during the period	7,297,000
Liabilities settled during the period	(381,000)	(316,000)
Accretion expense	379,000	735,000
Revision of estimated cash flows	1,448,000	0
Balance at end of period	24,232,000	15,489,000
Bellaire
Segment Reporting Information [Line Items]
Fair value of trust assets	7,200,000		5,000,000
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of period	16,926,000	16,416,000
Liabilities acquired during the period	0
Liabilities settled during the period	(1,128,000)	(1,243,000)
Accretion expense	1,183,000	1,161,000
Revision of estimated cash flows	606,000	592,000
Balance at end of period	$ 17,587,000	$ 16,926,000

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Asset Retirement Obligation

-URI http://asc.fasb.org/extlink&oid=6505190

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- Definition

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)(3)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

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Carrying amount of an asset that is legally restricted for purposes of settling an asset retirement obligation.

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Amount of asset retirement obligations incurred during the period.

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Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

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Amount of the Increase or Decrease in the amount of the asset retirement obligation during the current period for changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

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