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Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 40,097	$ 31,200
Goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues	16,086	17,021
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues	24,011	14,179
Operating segments | Coal Mining
Disaggregation of Revenue [Line Items]
Revenues	16,750	17,597
Operating segments | NAMining
Disaggregation of Revenue [Line Items]
Revenues	10,775	10,213
Operating segments | Minerals Management
Disaggregation of Revenue [Line Items]
Revenues	12,686	3,476
Unallocated Items
Disaggregation of Revenue [Line Items]
Revenues	543	0
Eliminations
Disaggregation of Revenue [Line Items]
Revenues	$ (657)	$ (86)

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+ References

No definition available.

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Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
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-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045

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