Commitments and Contingencies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 31, 2018
Commitments and Contingencies [Line Items]
Asset retirement obligation depreciation and accretion	$ 9,100,000	$ 8,000,000.0	$ 8,700,000
Liability reserve for insurance claims	39,900,000	48,300,000
Outstanding letters of credit	32,900,000
Surety bonds	395,100,000
Due from affiliate	11,300,000
Guarantee of affiliate's obligations				$ 15,500,000
Purchase commitments for property, plant and equipment	176,300,000
Capital expenditures	$ 106,700,000	79,300,000	$ 83,700,000
Percentage of employees represented in labor union	11.00%
Woodville
Commitments and Contingencies [Line Items]
Collective bargaining agreement expiration month year	2022-06
Magnesia Specialties
Commitments and Contingencies [Line Items]
Percentage of employees represented in labor union	100.00%
Collective bargaining agreement expiration month year	2023-08
Capital Addition Purchase Commitments
Commitments and Contingencies [Line Items]
Purchase commitments for property, plant and equipment	$ 93,400,000
Energy And Service Contracts
Commitments and Contingencies [Line Items]
Purchase commitments for property, plant and equipment	82,900,000
Interest-only loan
Commitments and Contingencies [Line Items]
Due from affiliate	$ 6,000,000.0	$ 6,000,000.0
Maturity date	Dec. 31, 2022	Dec. 31, 2022
Revolving Line Of Credit Expires In August 2015
Commitments and Contingencies [Line Items]
Line of credit maturity period	2020-03
Revolving Facility
Commitments and Contingencies [Line Items]
Outstanding letters of credit	$ 2,300,000	$ 2,300,000