Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets, Net [Abstract]
Net Balance	$ 35,330	$ 37,902
Amortization of intangible assets	2,572	2,614
Expected annual amortization expense, 2022	3,100
Coal supply agreement
Finite-Lived Intangible Assets, Net [Abstract]
Gross Carrying Amount	84,200	84,200
Accumulated Amortization	(48,870)	(46,298)
Net Balance	35,330	$ 37,902
NACoal
Finite-Lived Intangible Assets, Net [Abstract]
Expected annual amortization expense, 2021	3,100
Expected annual amortization expense, 2023	3,200
Expected annual amortization expense, 2024	3,100
Expected annual amortization expense, 2025	$ 3,200

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount of amortization for assets, excluding financial assets and goodwill, lacking physical substance with finite life expected to be recognized in fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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