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Fair Value Disclosure (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2022 | Jun. 30, 2021 | Jun. 30, 2022 | Jun. 30, 2021 | May 02, 2022 | Dec. 31, 2020
Coal Mining |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Fair value gained from contract termination |  |  |  |  | $ 4.1 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value measurements, recurring |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Unrealized (loss) gain on equity securities | $ (0.4) | $ 0.6 | $ 0.8 | $ 1.1 |  |
Significant Unobservable Inputs (Level 3) | Fair value measurements, nonrecurring | Coal Mining |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Fair value gained from contract termination |  |  |  |  | 4.1 |
Investment | 5.0 |  | 5.0 |  | $ 5.0 |
Fair value of privately-held company | 12.8 |  | 12.8 |  |  |
Bellaire | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value measurements, recurring |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Unrealized (loss) gain on equity securities | $ (1.5) | $ 0.7 | $ (2.1) | $ 1.0 |  |
Carrying Value | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value measurements, recurring |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Equity securities cost |  |  |  |  |  | $ 2.0
Carrying Value | Bellaire | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value measurements, recurring |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Equity securities cost |  |  |  |  |  | $ 5.0

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