Derivative Financial Instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Fair values of derivative instruments, balance sheet
Commodity futures, asset derivatives (in dollars)	$ 3,620,000		$ 1,010,000
Commodity futures, liability derivatives (in dollars)	(1,526,000)		4,487,000
Gains or losses on derivative instruments, statement of income
Amount of gain recognized in income on related hedged items	296,000	221,000
Gain (loss) excluded from hedge effectiveness testing	(183,000)	2,300,000
Other current assets
Fair values of derivative instruments, balance sheet
Fair value of derivatives including required margin deposits	5,400,000		3,600,000
Designated
Gains or losses on derivative instruments, statement of income
Amount of gain (loss) recognized in income on related hedged items	(1,504,000)	(5,492,000)
Futures | Aluminum | Long
Commodity contract commitments
Commodity contract (in MT/Lbs)	2,625
Futures | Aluminum | Short
Commodity contract commitments
Commodity contract (in MT/Lbs)	1,525
Futures | Copper | Long
Commodity contract commitments
Commodity contract (in MT/Lbs)	4,260
Futures | Copper | Short
Commodity contract commitments
Commodity contract (in MT/Lbs)	15,014
Futures | Silver | Short
Commodity contract commitments
Commodity contract (in MT/Lbs)	343
Commodity contract | Futures | Designated | Costs of goods sold
Gains or losses on derivative instruments, statement of income
Amount of gain (loss) recognized in income on related hedged items	1,617,000	8,046,000
Commodity contract | Futures | Designated | Other current assets
Fair values of derivative instruments, balance sheet
Commodity futures, asset derivatives (in dollars)	2,154,000		658,000
Commodity futures, liability derivatives (in dollars)	(1,048,000)		1,886,000
Commodity contract | Futures | Not designated | Costs of goods sold
Gains or losses on derivative instruments, statement of income
Amount of gain recognized in income on derivatives	7,956,000	2,401,000
Commodity contract | Futures | Not designated | Other current assets
Fair values of derivative instruments, balance sheet
Commodity futures, asset derivatives (in dollars)	1,466,000		352,000
Commodity futures, liability derivatives (in dollars)	(478,000)		2,601,000
Interest rate swap
Commodity contract commitments
Number of derivative instruments	0
Forward exchange contracts on foreign currency
Commodity contract commitments
Number of derivative instruments	0
Firm commitments | Designated | Costs of goods sold
Gains or losses on derivative instruments, statement of income
Amount of gain (loss) recognized in income on related hedged items	984,000	1,316,000
Inventory | Designated | Costs of goods sold
Gains or losses on derivative instruments, statement of income
Amount of gain (loss) recognized in income on related hedged items	$ (2,488,000)	$ (6,808,000)