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Investments - Investments Reported in Other Current and Non-Current Assets (Detail) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Equity Method Investments [Line Items] |  |
Assets | $ 74.5 | $ 79.1
Funding Commitments | 0.0 |
Chem-Mod LLC [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 4.0 | 4.0
Funding Commitments | 0.0 |
Chem-Mod International LLC [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 2.0 | 2.0
Funding Commitments | 0.0 |
Clean-Coal Investments [Member] | Controlling Interest [Member] | 14 2009 Era Clean Coal Plants [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 8.9 | 10.2
Funding Commitments | 0.0 |
Clean-Coal Investments [Member] | Controlling Interest [Member] | 19 2011 Era Clean Coal Plants [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 54.9 | 58.5
Funding Commitments | 0.0 |
Clean-Coal Investments [Member] | Noncontrolling Interests [Member] | One 2011 Era Clean Coal Plant [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 0.5 | 0.6
Funding Commitments | 0.0 |
Other Investments [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 4.2 | $ 3.8
Funding Commitments | $ 0.0 |

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