Derivative Financial Instruments - Summary of Classification of Fair Value on Derivatives Designated as Cash Flow Hedges (Details) - Gold zero cost collars - Designated as Hedging Instrument $ in Thousands	Sep. 30, 2019 USD ($)
Prepaid expenses and other
Derivatives, Fair Value [Line Items]
Fair value of derivative asset	$ 1,497
Accrued liabilities and other
Derivatives, Fair Value [Line Items]
Fair value of derivative liability	$ 0