Revenue - Performance Obligations (Details) $ in Thousands	Mar. 31, 2020 USD ($)
Revenue from Contract with Customer [Abstract]
Estimated coal revenues	$ 956,231
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-04-01
Revenue from Contract with Customer [Abstract]
Estimated coal revenues	$ 324,629
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation, period	9 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue from Contract with Customer [Abstract]
Estimated coal revenues	$ 264,690
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue from Contract with Customer [Abstract]
Estimated coal revenues	$ 190,644
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue from Contract with Customer [Abstract]
Estimated coal revenues	$ 130,268
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Revenue from Contract with Customer [Abstract]
Estimated coal revenues	$ 46,000
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Revenue from Contract with Customer [Abstract]
Estimated coal revenues	$ 0
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation, period	1 year