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Investments - Additional Information (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]      
Ownership interest held 50.00%    
Total assets of variable interest entities $ 22,331.4 $ 19,634.8 $ 16,334.0
Total liabilities of variable interest entities $ 16,098.7 14,419.3  
Ownership interest held 50.00%    
Variable Interest Entity, Not Primary Beneficiary [Member]      
Schedule of Equity Method Investments [Line Items]      
Ownership interest held 12.00%    
Chem-Mod Clean-Coal Venture - U.S. and Canadian Operations [Member]      
Schedule of Equity Method Investments [Line Items]      
Ownership interest held 46.50%    
Chem-Mod Clean-Coal Venture [Member]      
Schedule of Equity Method Investments [Line Items]      
Total revenues of limited liability companies $ 74.3 81.9  
Total expenses of limited liability companies 10.6 17.5  
Chem-Mod Clean-Coal Venture [Member] | VIE [Member]      
Schedule of Equity Method Investments [Line Items]      
Total assets of variable interest entities 19.0 16.3  
Total liabilities of variable interest entities $ 4.4 $ 2.8  
Chem-Mod International LLC [Member]      
Schedule of Equity Method Investments [Line Items]      
Ownership interest held 31.50%