Commitments and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Commitments and Contingencies [Line Items]
Asset retirement obligation depreciation and accretion		$ 14,500,000	$ 9,100,000	$ 8,000,000.0
Liability reserve for insurance claims		37,700,000	39,900,000
Outstanding letters of credit		32,000,000.0
Surety bonds		390,200,000
Due from affiliate		8,400,000
Guarantee of affiliate's obligations		12,500,000
Purchase commitments for property, plant and equipment		225,200,000
Capital expenditures		$ 77,000,000.0	106,700,000	$ 79,300,000
Percentage of employees represented in labor union		10.00%
Woodville
Commitments and Contingencies [Line Items]
Collective bargaining agreement expiration month year		2022-06
Magnesia Specialties
Commitments and Contingencies [Line Items]
Collective bargaining agreement expiration month year		2023-08
Capital Addition Purchase Commitments
Commitments and Contingencies [Line Items]
Purchase commitments for property, plant and equipment		$ 102,800,000
Energy And Service Contracts
Commitments and Contingencies [Line Items]
Purchase commitments for property, plant and equipment		122,400,000
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	2022-03
Revolving Facility
Commitments and Contingencies [Line Items]
Outstanding letters of credit		$ 2,600,000	$ 2,300,000