Derivative Instruments - Additional Information (Details) oz in Thousands, lb in Millions	9 Months Ended
	Sep. 30, 2015 lb oz $ / lb $ / oz
Gold Contracts - Embedded Derivative
Provisional Gold and Copper Sales - Embedded derivatives
Provisional pricing quantity sales | oz	203
Average price, subject to final pricing | $ / oz	1,114
Copper Contracts - Embedded Derivative
Provisional Gold and Copper Sales - Embedded derivatives
Provisional pricing quantity sales | lb	138
Average price, subject to final pricing	2.30