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Investments - Investments Reported in Other Current and Non-Current Assets (Detail) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]    
Assets $ 25.9 $ 40.0
Funding Commitments 0.2  
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] | Twenty 2011 Era Clean Coal Plants [Member]    
Schedule of Equity Method Investments [Line Items]    
Assets 15.9 29.2
Funding Commitments 0.2  
Clean-Coal Investments [Member] | Noncontrolling Interests [Member] | One 2011 Era Clean Coal Plant [Member]    
Schedule of Equity Method Investments [Line Items]    
Assets 0.2 0.3
Other Investments [Member]    
Schedule of Equity Method Investments [Line Items]    
Assets $ 3.8 $ 4.5