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Fair Value Measurements - Additional Information (Detail)
 6 Months Ended
Jun. 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 $ 1,256
Maximum [Member]  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Gold forward-looking price range $ 2,839