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Investments - Clean Coal Investments - Additional Information (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2020 USD ($) Entity Plant
Chem-Mod LLC [Member] |
Schedule of Equity Method Investments [Line Items] |
Number of clean coal production plants owned | Plant | 35
Chem-Mod LLC [Member] | 2009 Era Plants [Member] |
Schedule of Equity Method Investments [Line Items] |
Number of clean coal production plants owned | Plant | 14
Chem-Mod LLC [Member] | 2011 Era Plants [Member] |
Schedule of Equity Method Investments [Line Items] |
Number of clean coal production plants owned | Plant | 21
Clean-Coal Investments [Member] |
Schedule of Equity Method Investments [Line Items] |
Number of variable interest entities | Entity | 1
Clean-Coal Investments [Member] | VIE [Member] |
Schedule of Equity Method Investments [Line Items] |
Total assets of limited liability companies | $ 36.6
Total liability of limited liability companies | 36.0
Total revenues of limited liability companies | 35.2
Total expenses of limited liability companies | $ 43.9

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