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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% |  |

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