Asset Impairment and Restructuring - Restructuring Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Restructuring Cost and Reserve [Line Items]
Total restructuring expense	$ 22,435	$ 0	$ 22,435	$ 0
Allowance for obsolete materials and supplies inventory	807
Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Restructuring Cost and Reserve [Line Items]
Severance and employee-related benefits	20,553		20,553
Other costs	1,882		1,882
Total restructuring expense	22,435		22,435
Deferred longwall move expenses	668		668
Allowance for advanced mining royalties	407		407
Allowance for obsolete materials and supplies inventory	807		807
All Other | Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Restructuring Cost and Reserve [Line Items]
Total restructuring expense	1,993		1,993
CAPP - Thermal | Operating Segments | Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Restructuring Cost and Reserve [Line Items]
Total restructuring expense	2,310		2,310
NAPP | Operating Segments | Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Restructuring Cost and Reserve [Line Items]
Total restructuring expense	$ 18,132		$ 18,132