Asset Impairment and Restructuring - Narrative (Details)	3 Months Ended				9 Months Ended
	Sep. 30, 2020 USD ($) asset_group	Jun. 30, 2020 USD ($) asset_group	Mar. 31, 2020 USD ($) asset_group	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment			$ 33,709,000
Asset impairment related to write-off of prepaid purchased coal				$ 32,000		$ 5,858,000
Restructuring expense				$ 0		$ 0
Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Impaired Long-Lived Assets Held and Used [Line Items]
Restructuring expense	$ 55,000				$ 22,490,000
Operating Segments
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		$ 161,738,000
All Other
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		$ 5,000
All Other | Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Impaired Long-Lived Assets Held and Used [Line Items]
Restructuring expense	$ 450,000				2,443,000
CAPP - Met
Impaired Long-Lived Assets Held and Used [Line Items]
Number of assets impaired | asset_group	3	3	4
CAPP - Met | Operating Segments
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	$ 0	$ 0	$ 32,951,000
CAPP - Thermal
Impaired Long-Lived Assets Held and Used [Line Items]
Number of assets impaired | asset_group	4	4	3
CAPP - Thermal | Operating Segments
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		$ 17,385,000	$ 758,000
CAPP - Thermal | Operating Segments | Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Impaired Long-Lived Assets Held and Used [Line Items]
Restructuring expense	$ (484,000)				1,826,000
NAPP | Operating Segments
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		144,348,000
NAPP | Operating Segments | Strategic Actions with Respect to Two Thermal Coal Mining Complexes
Impaired Long-Lived Assets Held and Used [Line Items]
Restructuring expense	89,000				$ 18,221,000
Mineral rights
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		18,605,000	21,825,000
Property, plant and equipment
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		134,480,000	6,066,000
Property, plant and equipment | Operating Segments
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	3,516,000
Property, plant and equipment | CAPP - Thermal | Operating Segments
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	219,000
Property, plant and equipment | NAPP | Operating Segments
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	$ 3,297,000
Acquired mine permits, net
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		$ 8,653,000	$ 5,818,000