Commitments and Contingencies - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Settlement proceeds	$ 1,800,000					$ 22,800,000
Throughput obligation		$ 0		$ 0		$ 0
Coal royalty expense		$ 12,000,000.0	$ 19,700,000	$ 39,800,000	$ 75,700,000