Fair Value Measurements - Additional Information (Detail)	9 Months Ended
	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