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Commitments and Contingencies - Additional Information (Detail) - USD ($) | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017 | Sep. 30, 2017
Commitments and Contingencies [Line Items] |  |  |
Guarantee of affiliate's obligations | $ 15,500,000 |  |
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 |  |
Interest-only loan |  |  |
Commitments and Contingencies [Line Items] |  |  |
Due from affiliate | $ 6,000,000 | $ 6,000,000 | $ 6,000,000
Due date | Dec. 31, 2019 |  |
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 date | 2020-03 |  |

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