Derivative Financial Instruments - Summary of Pre-tax Gains (Losses) On Derivatives Designated as Cash Flow Hedges (Details) - Gold zero cost collars - Designated as Hedging Instrument - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gains (losses) recognized in OCI - effective portion:	$ 1,497	$ 0	$ 1,497	$ 0
Gains (losses) reclassified from AOCI into net income - effective portion:	$ 0	$ 0	$ 0	$ 0