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Investments - Investments Reported in Other Current and Non-Current Assets (Detail) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]    
Assets $ 69.9 $ 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 7.6 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 51.1 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 4.7 $ 3.8
Funding Commitments $ 0.0