Derivatives - Effects of derivatives on measures of financial performance (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2023	Mar. 31, 2022
Coal Contract \| Change in fair value of coal derivatives and coal trading activities, net
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives	$ 1,462	$ (15,519)
Coal Contract \| Other operating (income) expense, net
Derivative Instruments, Gain (Loss) [Line Items]
Realized Gain (Loss) on Derivatives	2,667	(9,074)
Heating oil - diesel purchases \| Other operating (income) expense, net
Derivative Instruments, Gain (Loss) [Line Items]
Heating oil realized and unrealized gains and losses	$ (1,282)	$ 6,801

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No definition available.

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- Definition

Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4CC
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL109998890-113959

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (e)
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5624171-113959

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- Definition

The difference between the book value and the sale price of options, swaps, futures, forward contracts, and other derivative instruments. This element refers to the gain (loss) included in earnings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.13(h))
-URI https://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

+ References

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