Derivative Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Fair values of derivative instruments, balance sheet
Commodity futures net liability/asset (in dollars)	$ 310		$ 310		$ 4,988
Gain (loss) on commodity futures contracts (in dollars)	$ 1,422	$ 2,477	$ 4,345	$ 4,408
Aluminum | Long
Commodity futures contract commitments
Commodity futures contract (in MT/Lbs)	3,500		3,500
Aluminum | Short
Commodity futures contract commitments
Commodity futures contract (in MT/Lbs)	3,450		3,450
Copper | Long
Commodity futures contract commitments
Commodity futures contract (in MT/Lbs)	6,951		6,951
Copper | Short
Commodity futures contract commitments
Commodity futures contract (in MT/Lbs)	10,115		10,115
Silver | Short
Commodity futures contract commitments
Commodity futures contract (in MT/Lbs)	343		343