Derivatives (Schedule of Price Risk Derivatives) (Details) T in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) T	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Coal sales
Derivative [Line Items]
Derivatives held (in tons)	193
Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	82
Coal Contract | Designated as Hedging Instrument
Derivative [Line Items]
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) | $	$ (392)	$ 9,481	$ (8,205)
Coal sales | Coal Contract | Designated as Hedging Instrument | Coal sales
Derivative [Line Items]
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) | $	(1,850)	10,167	(10,912)
Coal purchases | Coal Contract | Designated as Hedging Instrument | Coal purchases
Derivative [Line Items]
Gains (Losses) Reclassified from Other Comprehensive Income into Income (Effective Portion) | $	$ 1,458	$ (686)	$ 2,707
2021 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	193
2021 | Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	82
2021 | Coal sales
Derivative [Line Items]
Derivatives held (in tons)	0
2021 | Coal purchases
Derivative [Line Items]
Derivatives held (in tons)	0