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Investments - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
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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 |

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