Commitments and Contingencies - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Commitments and Contingencies [Line Items]
Guarantee of affiliate's obligations	$ 25,000,000
Due from affiliate	$ 6,000,000	$ 6,000,000	$ 6,000,000
Percentage of employees represented in labor union	10.00%
Woodville
Commitments and Contingencies [Line Items]
Collective bargaining agreement expiration month year	2018-05
Magnesia Specialties
Commitments and Contingencies [Line Items]
Percentage of employees represented in labor union	100.00%
Collective bargaining agreement expiration month year	2019-08