Investments - Clean Coal Investments - Additional Information (Detail) $ in Millions	12 Months Ended
	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