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Investments - Investments Reported in Other Current and Non-Current Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Assets	$ 14.1	$ 22.4
Funding Commitments	0.5
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
Clean-Coal Investments [Member] | Controlling Interest [Member] | Twenty 2011 Era Clean Coal Plants [Member]
Schedule of Equity Method Investments [Line Items]
Assets	2.6	12.4
Other Investments [Member]
Schedule of Equity Method Investments [Line Items]
Assets	5.5	$ 3.9
Funding Commitments	$ 0.5

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