Business Segments (Concentration Risk) (Details) - Percentage of Consolidated Revenue - Customer concentration risk	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Coal Mining customer \| Coal Mining customer
Segment Reporting Information [Line Items]
Percentage of Consolidated Revenue	55.00%	48.00%
NAMining customer \| NAMining customer
Segment Reporting Information [Line Items]
Percentage of Consolidated Revenue	19.00%	21.00%
Minerals Management lessee \| Minerals Management lessee (less than in 2020)
Segment Reporting Information [Line Items]
Percentage of Consolidated Revenue	10.00%	12.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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