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Commitments and Contingencies - Additional Information (Detail) - USD ($) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018
Commitments and Contingencies [Line Items] |  |
Guarantee of affiliate's obligations | $ 15,500,000 |
Due from affiliate | 12,800,000 |
Outstanding letters of credit | $ 36,100,000 |
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 | 2019-08 |
Revolving Facility |  |
Commitments and Contingencies [Line Items] |  |
Maturity date | Dec. 05, 2023 |
Outstanding letters of credit | $ 2,300,000 | $ 2,300,000
Interest-only loan |  |
Commitments and Contingencies [Line Items] |  |
Due from affiliate | $ 6,000,000.0 | $ 6,000,000.0
Maturity date | Dec. 31, 2022 |
Interest-only loan | London Interbank Offered Rate(LIBOR) |  |
Commitments and Contingencies [Line Items] |  |
Basis spread on variable rate | 1.75% |
Revolving Line Of Credit Expires In August 2015 |  |
Commitments and Contingencies [Line Items] |  |
Line of credit maturity period | 2020-03 |

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