Derivative Financial Instruments - Summary of Derivative Cash Flow Hedges (Details) - Designated as Hedging Instrument	9 Months Ended
	Sep. 30, 2019 oz $ / oz
Gold Put Options - 2019
Derivative [Line Items]
Average gold strike price per ounce | $ / oz	1,405
Notional ounces | oz	63,000
Gold Put Options - 2020
Derivative [Line Items]
Average gold strike price per ounce | $ / oz	1,408
Notional ounces | oz	96,000
Gold Call Options - 2019
Derivative [Line Items]
Average gold strike price per ounce | $ / oz	1,798
Notional ounces | oz	63,000
Gold Call Options - 2020
Derivative [Line Items]
Average gold strike price per ounce | $ / oz	1,803
Notional ounces | oz	96,000