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Derivatives and Fair Value Measurements- Changes in Recurring Level 3 Net Financial Assets (Details) - Coal Trading - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |
Beginning of period | $ 2.5 | $ 4.0 | $ 4.0
Impairment loss included in earnings | (2.0) | (1.7) | 0.0
Purchases | 0.0 | 0.2 | 0.0
Exchange | (0.5) | 0.0 | 0.0
End of period | $ 0.0 | $ 2.5 | $ 4.0

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- Definition Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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