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Investments - Additional Information (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]    
Ownership interest held 50.00%  
Chem-Mod Clean-Coal Venture [Member]    
Schedule of Equity Method Investments [Line Items]    
Total assets of variable interest entities $ 14.0 $ 11.1
Total liabilities of variable interest entities 1.4 0.7
Total revenues of limited liability companies 73.7 64.4
Total expenses of limited liability companies $ 4.1 $ 2.3
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 International LLC [Member]    
Schedule of Equity Method Investments [Line Items]    
Ownership interest held 31.50%  
C-Quest Technology LLC and C-Quest Technologies International LLC [Member]    
Schedule of Equity Method Investments [Line Items]    
Ownership interest held 12.00%  
Option to acquire additional interest, percent 15.00%  
Option to acquire additional interest, total price $ 7.5  
End date for acquiring additional interest Feb. 15, 2019