Investments - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
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	$ 11.1	$ 11.1
Total liabilities of variable interest entities	0.7	0.8
Total revenues of limited liability companies	64.4	63.5
Total expenses of limited liability companies	$ 2.3	$ 2.4
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