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Investments - Investments Reported in Other Current and Non-Current Assets (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Equity Method Investments [Line Items] |  |
Assets | $ 79.1 | $ 93.7
Funding Commitments | 0.4 |
Chem-Mod LLC [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 4.0 | 4.0
Chem-Mod International LLC [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 2.0 | 2.0
Clean-Coal Investments [Member] | Controlling Interest [Member] | 14 2009 Era Clean Coal Plants [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 10.2 | 14.3
Clean-Coal Investments [Member] | Controlling Interest [Member] | 19 2011 Era Clean Coal Plants [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 58.5 | 69.0
Clean-Coal Investments [Member] | Noncontrolling Interests [Member] | One 2011 Era Clean Coal Plant [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 0.6 | 0.7
Other Investments [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 3.8 | $ 3.7
Funding Commitments | $ 0.4 |

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