Asset Impairment (Details)		3 Months Ended
	Feb. 29, 2020 asset_group	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		$ 33,709,000	$ 0
CAPP - Met
Impaired Long-Lived Assets Held and Used [Line Items]
Number of assets impaired | asset_group	4
Asset impairment		32,951,000
CAPP - Thermal
Impaired Long-Lived Assets Held and Used [Line Items]
Number of assets impaired | asset_group	3
Asset impairment		758,000
Mineral rights
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		21,825,000
Property, plant and equipment
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		5,981,000
Acquired mine permits, net
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		5,818,000
Asset retirement obligation asset
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		$ 85,000