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Fair Value Measurements - Additional Information (Detail) | 9 Months Ended
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Sep. 30, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
Liquid investments, original maturities | 90 days
Minimum [Member] |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
Gold forward-looking price range | $ 1,193
Maximum [Member] |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
Gold forward-looking price range | $ 2,693

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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