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Investments - Clean Coal Investments - Additional Information (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($) Entity Plant
2009 Era Plants [Member] |
Schedule of Equity Method Investments [Line Items] |
Number of clean coal production plants seeking and negotiating for long term production contract | 2
2011 Era Plants [Member] |
Schedule of Equity Method Investments [Line Items] |
Number of clean coal production plants seeking and negotiating for long term production contract | 1
Chem-Mod LLC [Member] |
Schedule of Equity Method Investments [Line Items] |
Number of clean coal production plants owned | 34
Chem-Mod LLC [Member] | 2009 Era Plants [Member] |
Schedule of Equity Method Investments [Line Items] |
Number of clean coal production plants owned | 14
Chem-Mod LLC [Member] | 2011 Era Plants [Member] |
Schedule of Equity Method Investments [Line Items] |
Number of clean coal production plants owned | 20
Long Term Production Contracts [Member] |
Schedule of Equity Method Investments [Line Items] |
Number of clean coal production plants owned | 31
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 | $ | $ 23.0
Total liability of limited liability companies | $ | 21.1
Total revenues of limited liability companies | $ | 66.2
Total expenses of limited liability companies | $ | $ 81.1

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