Commitments and Contingencies - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Settlement proceeds	$ 1,800,000
Throughput obligation		$ 0		$ 0
Coal royalty expense		$ 12,700,000	$ 14,200,000