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Investments - Investments Reported in Other Current and Non-Current Assets (Detail) - USD ($) $ in Millions | Jun. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Schedule of Equity Method Investments [Line Items] |  |
Assets | $ 17.1 | $ 22.4
Funding Commitments | 1.0 |
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] | Noncontrolling Interests [Member] | One 2011 Era Clean Coal Plant [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 0.1 | 0.1
Clean-Coal Investments [Member] | Controlling Interest [Member] | Twenty 2011 Era Clean Coal Plants [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 6.1 | 12.4
Other Investments [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | 4.9 | $ 3.9
Funding Commitments | $ 1.0 |

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- Definition This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=109237563&loc=d3e33749-111570 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 25 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8813-108599 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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