Asset Impairment and Restructuring - Restructuring Expense (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Restructuring Cost and Reserve [Line Items]
Total restructuring expense			$ 0		$ 0
Allowance for obsolete materials and supplies inventory		$ 807,000
Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Restructuring Cost and Reserve [Line Items]
Severance and employee-related benefits	$ 55,000			$ 20,608,000
Other costs	0			1,882,000
Total restructuring expense	55,000			22,490,000
Deferred longwall move expenses				668,000
Allowance for advanced mining royalties				407,000
Allowance for obsolete materials and supplies inventory				807,000
All Other | Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Restructuring Cost and Reserve [Line Items]
Total restructuring expense	450,000			2,443,000
CAPP - Thermal | Operating Segments | Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Restructuring Cost and Reserve [Line Items]
Total restructuring expense	(484,000)			1,826,000
NAPP | Operating Segments | Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Restructuring Cost and Reserve [Line Items]
Total restructuring expense	$ 89,000			$ 18,221,000