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Investments - Investments Reported in Other Current and Non-Current Assets (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]    
Assets $ 79.1 $ 93.7
Funding Commitments 0.4  
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] | 14 2009 Era Clean Coal Plants [Member]    
Schedule of Equity Method Investments [Line Items]    
Assets 10.2 14.3
Clean-Coal Investments [Member] | Controlling Interest [Member] | 19 2011 Era Clean Coal Plants [Member]    
Schedule of Equity Method Investments [Line Items]    
Assets 58.5 69.0
Clean-Coal Investments [Member] | Noncontrolling Interests [Member] | One 2011 Era Clean Coal Plant [Member]    
Schedule of Equity Method Investments [Line Items]    
Assets 0.6 0.7
Other Investments [Member]    
Schedule of Equity Method Investments [Line Items]    
Assets 3.8 $ 3.7
Funding Commitments $ 0.4