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Investments - Clean Coal Investments - Additional Information (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2018
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 | $ $ 31.4
Total liability of limited liability companies | $ 30.0
Total revenues of limited liability companies | $ 90.1
Total expenses of limited liability companies | $ $ 110.2