Revenue Recognition (Narrative) (Details) - Coal T in Millions	Dec. 31, 2019 T
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation (in tons)	67.4
Remaining performance obligations, fixed price contracts (in tons)	61.6
Remaining performance obligation, variable price contracts (in tons)	5.8
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation (in tons)	69.0
Remaining performance obligations, fixed price contracts (in tons)	64.9
Remaining performance obligation, variable price contracts (in tons)	4.1