Investments - Investments Reported in Other Current and Non-Current Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Assets	$ 22.4	$ 40.0
Funding commitments	0.9
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	12.4	29.2
Funding commitments	0.9
Clean-Coal Investments [Member] | Noncontrolling Interests [Member] | One 2011 Era Clean Coal Plant [Member]
Schedule of Equity Method Investments [Line Items]
Assets	0.1	0.3
Other Investments [Member]
Schedule of Equity Method Investments [Line Items]
Assets	$ 3.9	$ 4.5