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Nature of Operations and Basis of Presentation (Narrative) (Details) $ in Thousands, T in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2020 USD ($) T | Sep. 30, 2019 | Sep. 30, 2020 USD ($) segment | Dec. 31, 2019 USD ($) T | Dec. 31, 2018 T | Jul. 01, 2019 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract] |  |  |  |  |  |
Number of operating segments | segment |  |  | 3 |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |
Contractual obligation amount included in other noncurrent assets | $ | $ 39,108 |  | $ 39,108 | $ 32,133 |  |
Coal Mining |  |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |
Historical lignite coal annual delivery |  |  |  | 34.6 |  |
Outstanding securitized accounts receivable | $ | 2,300 |  | $ 2,300 |  |  |
Coal Mining | Caddo Creek |  |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |
Historical lignite coal annual delivery |  |  |  | 0.2 |  |
Coal Mining | Sabine |  |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |
Historical lignite coal annual delivery |  |  |  | 2.6 | 3.8 |
Decrease in coal delivered |  |  | 43.00% |  |  |
NAMining | Lithium Nevada Corp. |  |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |
Term of contractual obligation |  | 20 years |  |  |  |
Contractual obligation amount provided | $ |  |  |  |  |  | $ 3,500
Contractual obligation amount included in other noncurrent assets | $ | $ 3,000 |  | $ 3,000 | $ 1,500 |  |
Minimum | Coal Mining | Sabine |  |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |
Historical lignite coal annual delivery |  |  |  | 2.5 |  |
Expected future lignite coal annual delivery | 1.4 |  |  |  |  |
Maximum | Coal Mining | Sabine |  |  |  |  |  |
Long-term Purchase Commitment [Line Items] |  |  |  |  |  |
Historical lignite coal annual delivery |  |  |  | 3.5 |  |
Expected future lignite coal annual delivery | 1.7 |  |  |  |  |

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