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Fair Value Measurements - Additional Information (Detail) - USD ($)
 6 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Liquid investments, original maturities 90 days  
Perpetual growth rate for deferred consideration 1.50% 1.50%
Discounted rate for deferred consideration 10.00%  
Minimum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gold forward-looking price range $ 1,417 $ 1,294
Maximum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gold forward-looking price range $ 2,201 $ 2,621