Inventories, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Inventory [Line Items]
Total inventories, net	$ 184,445	$ 162,659
Coal
Inventory [Line Items]
Raw coal	33,016	30,274
Saleable coal	123,547	105,092
Materials, supplies and other, net
Inventory [Line Items]
Materials, supplies and other, net	$ 27,882	$ 27,293