Segment Information - Revenue from External Customers (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenues from External Customers and Long-Lived Assets [Line Items]
Coal revenues	$ 399,954	$ 523,987	$ 1,278,935	$ 1,784,775
Export coal revenues
Revenues from External Customers and Long-Lived Assets [Line Items]
Coal revenues	209,046	$ 261,815	718,058	967,959
India
Revenues from External Customers and Long-Lived Assets [Line Items]
Coal revenues	$ 57,740		174,182	$ 228,754
Brazil
Revenues from External Customers and Long-Lived Assets [Line Items]
Coal revenues			$ 133,501
Geographic Concentration Risk | Export coal revenues | Revenues
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk percentage	52.00%	50.00%	56.00%	54.00%