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Fair Value Measurements - Additional Information (Detail) - USD ($)
 3 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Liquid investments, original maturities 90 days  
Growth rate for deferred consideration 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 $ 1,303 $ 1,294
Maximum [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gold forward-looking price range $ 2,394 $ 2,621