Investments - Investments Reported in Other Current and Non-Current Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Assets	$ 65.1	$ 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	6.4	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	47.4	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	0.4	0.6
Funding Commitments	0.0
Other Investments [Member]
Schedule of Equity Method Investments [Line Items]
Assets	4.9	$ 3.8
Funding Commitments	$ 0.0