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Investments - Investments Reported in Other Current and Non-Current Assets (Detail) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]    
Assets $ 54.9 $ 59.5
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] | Controlling Interest [Member] | Fourteen 2009 Era Clean Coal Plants [Member]    
Schedule of Equity Method Investments [Line Items]    
Assets 3.8 5.1
Clean-Coal Investments [Member] | Controlling Interest [Member] | Nineteen 2011 Era Clean Coal Plants [Member]    
Schedule of Equity Method Investments [Line Items]    
Assets 39.8 43.0
Clean-Coal Investments [Member] | Noncontrolling Interests [Member] | One 2011 Era Clean Coal Plant [Member]    
Schedule of Equity Method Investments [Line Items]    
Assets 0.4 0.4
Other Investments [Member]    
Schedule of Equity Method Investments [Line Items]    
Assets $ 4.9 $ 5.0