Derivative Financial Instruments (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Fair values of derivative instruments, balance sheet
Commodity futures, asset derivatives (in dollars)	$ 698,000		$ 698,000		$ 4,024,000
Commodity futures, liability derivatives (in dollars)	1,285,000		1,285,000		1,854,000
Gains or losses on derivative instruments, statement of income
Amount of loss recognized in income on related hedged items	312,000		199,000
Gain (loss) excluded from hedge effectiveness testing	(678,000)		392,000
Interest rate swap
Commodity contract commitments
Number of derivative instruments	0		0
Forward exchange contracts on foreign currency
Commodity contract commitments
Number of derivative instruments	0		0
Designated
Gains or losses on derivative instruments, statement of income
Amount of gain (loss) recognized in income on related hedged items	(1,372,000)		(7,581,000)
Designated | Commodity Futures
Fair values of derivative instruments, balance sheet
Commodity futures, asset derivatives (in dollars)	413,000		413,000
Commodity futures, liability derivatives (in dollars)	660,000		660,000
Gains or losses on derivative instruments, statement of income
Amount of gain (loss) recognized in income on related hedged items	381,000		7,773,000
Designated | Firm commitments
Gains or losses on derivative instruments, statement of income
Amount of gain (loss) recognized in income on related hedged items	(1,736,000)		877,000
Designated | Inventory
Gains or losses on derivative instruments, statement of income
Amount of gain (loss) recognized in income on related hedged items	364,000		(8,458,000)
Not designated | Commodity Futures
Fair values of derivative instruments, balance sheet
Commodity futures, asset derivatives (in dollars)	285,000		285,000		4,024,000
Commodity futures, liability derivatives (in dollars)	625,000		625,000		1,854,000
Gains or losses on derivative instruments, statement of income
Amount of gain (loss) recognized in income on derivatives	$ (2,836,000)	$ (9,085,000)	$ 3,793,000	$ (6,810,000)
Aluminum | Long | Commodity futures net
Commodity contract commitments
Commodity contract (in MT/Lbs)	2,300		2,300
Aluminum | Short | Commodity futures net
Commodity contract commitments
Commodity contract (in MT/Lbs)	2,225		2,225
Copper | Long | Commodity futures net
Commodity contract commitments
Commodity contract (in MT/Lbs)	2,710		2,710
Copper | Short | Commodity futures net
Commodity contract commitments
Commodity contract (in MT/Lbs)	8,448		8,448
Silver | Short | Commodity futures net
Commodity contract commitments
Commodity contract (in MT/Lbs)	686		686