Financial Instruments (Unaudited) Not Designated as Hedging Instruments (Details) oz in Thousands, lb in Millions	9 Months Ended
	Sep. 30, 2015 lb oz $ / lb $ / oz
Embedded derivatives in provisional sales contracts - Copper [Member] | Derivatives not designated as hedging instruments [Member]
Derivative [Line Items]
Open positions (in lbs except gold in oz) | lb	536
Average contract price	2.45
Average market price	2.34
Maturities through	Mar. 31, 2016
Embedded derivatives in provisional sales contracts - Gold [Member] | Derivatives not designated as hedging instruments [Member]
Derivative [Line Items]
Open positions (in lbs except gold in oz) | oz	149
Average contract price | $ / oz	1,125
Average market price | $ / oz	1,118
Maturities through	Dec. 31, 2015
Embedded derivatives in provisional purchase contracts - Copper [Member] | Derivatives not designated as hedging instruments [Member]
Derivative [Line Items]
Open positions (in lbs except gold in oz) | lb	87
Average contract price	2.45
Average market price	2.35
Maturities through	Jan. 31, 2016
Atlantic Copper [Member] | Copper forward contracts [Member] | Derivatives not designated as hedging instruments [Member]
Derivative [Line Items]
Open positions (in lbs except gold in oz) | lb	4
Average contract price	2.31
Maturities through	Nov. 30, 2015
Copper futures and swap contracts [Member] | Fair Value Hedging [Member] | Derivatives designated as hedging instruments [Member]
Derivative [Line Items]
Open positions (in lbs except gold in oz) | lb	59
Average contract price	2.46
Maturities through	Sep. 30, 2017