Financial Instruments (Unaudited) Not Designated as Hedging Instruments (Details)	Mar. 31, 2012 lb
Atlantic Copper's copper forward commodity contracts [Member] | Derivatives not designated as hedging instruments [Member]
Derivative [Line Items]
Derivative held (in lbs except gold in oz)	22,000,000
Average contract price	3.81
Maturities through	May 2012
Embedded derivatives in provisional sales contracts - Copper [Member] | Derivatives not designated as hedging instruments [Member]
Derivative [Line Items]
Derivative held (in lbs except gold in oz)	326,000,000
Average contract price	3.69
Average market price	3.82
Maturities through	August 2012
Embedded derivatives in provisional sales contracts - Gold [Member] | Derivatives not designated as hedging instruments [Member]
Derivative [Line Items]
Derivative held (in lbs except gold in oz)	53,000
Average contract price	1,690
Average market price	1,663
Maturities through	May 2012
Embedded derivatives in provisional purchase contracts - Copper [Member] | Derivatives not designated as hedging instruments [Member]
Derivative [Line Items]
Derivative held (in lbs except gold in oz)	181,000,000
Average contract price	3.69
Average market price	3.84
Maturities through	June 2012
Embedded derivatives in provisional purchase contracts - Molybdenum [Member] | Derivatives not designated as hedging instruments [Member]
Derivative [Line Items]
Derivative held (in lbs except gold in oz)	42,000
Average contract price	12.83
Average market price	12.44
Maturities through	April 2012
Fair Value Hedging [Member] | FMC's copper futures and swap contracts [Member] | Derivatives designated as hedging instruments [Member]
Derivative [Line Items]
Derivative held (in lbs except gold in oz)	68,000,000
Average contract price	3.72
Maturities through	April 2013