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Business Segments (Concentration Risk) (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Earnings of Unconsolidated Operations | Coal Mining
Segment Reporting Information [Line Items]
Percentage of Consolidated Revenue		60.00%
Percentage of Consolidated Revenue | Choctaw Generation Limited Partnership, LLLP | Coal Mining
Segment Reporting Information [Line Items]
Percentage of Consolidated Revenue	48.00%	60.00%
Percentage of Consolidated Revenue | CEMEX | NAMining
Segment Reporting Information [Line Items]
Percentage of Consolidated Revenue	21.00%	20.00%
Percentage of Consolidated Revenue | Ascent Resources | Minerals Management
Segment Reporting Information [Line Items]
Percentage of Consolidated Revenue	12.00%

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- Definition

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.

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