Revenue - Performance Obligations (Details) $ in Thousands	Dec. 31, 2021 USD ($)
Disaggregation of Revenue [Line Items]
Estimated coal revenues	$ 79,967
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Disaggregation of Revenue [Line Items]
Estimated coal revenues	$ 65,768
Estimated coal revenues, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Disaggregation of Revenue [Line Items]
Estimated coal revenues	$ 14,199
Estimated coal revenues, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Disaggregation of Revenue [Line Items]
Estimated coal revenues	$ 0
Estimated coal revenues, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Disaggregation of Revenue [Line Items]
Estimated coal revenues	$ 0
Estimated coal revenues, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Disaggregation of Revenue [Line Items]
Estimated coal revenues	$ 0
Estimated coal revenues, period	1 year