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Derivative Financial Instruments (Details)	12 Months Ended
Derivative Financial Instruments (Details)	Dec. 31, 2015 USD ($) lb MT	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Designated as hedging instrument
Gains or losses on derivative instruments, statement of income
Amount of gain (loss) recognized in income on related hedged items	$ 3,353,000	$ (1,923,000)	$ (7,317,000)
Commodity contract \| Futures
Fair values of derivative instruments, balance sheet
Commodity futures, asset derivatives (in dollars)	1,765,000	5,312,000
Commodity futures, liability derivatives (in dollars)	3,925,000	1,539,000
Commodity contract \| Futures \| Other current assets
Fair values of derivative instruments, balance sheet
Fair value of derivatives including required margin deposits	3,400,000	7,600,000
Commodity contract \| Futures \| Designated as hedging instrument
Gains or losses on derivative instruments, statement of income
Ineffectiveness - amount of gain (loss) recognized in income on related hedged items	90,000	206,000	(206,000)
Gain (loss) excluded from hedge effectiveness testing	(991,000)	649,000	398,000
Commodity contract \| Futures \| Designated as hedging instrument \| Cost of goods sold
Gains or losses on derivative instruments, statement of income
Amount of gain (loss) recognized in income on derivatives	(4,254,000)	2,778,000	7,509,000
Commodity contract \| Futures \| Designated as hedging instrument \| Other current assets
Fair values of derivative instruments, balance sheet
Commodity futures, asset derivatives (in dollars)	857,000	3,180,000
Commodity futures, liability derivatives (in dollars)	2,860,000	913,000
Commodity contract \| Futures \| Not designated as hedging instrument \| Cost of goods sold
Gains or losses on derivative instruments, statement of income
Amount of gain (loss) recognized in income on derivatives	16,261,000	14,988,000	2,097,000
Commodity contract \| Futures \| Not designated as hedging instrument \| Other current assets
Fair values of derivative instruments, balance sheet
Commodity futures, asset derivatives (in dollars)	908,000	2,132,000
Commodity futures, liability derivatives (in dollars)	$ 1,065,000	626,000
Commodity contract \| Futures \| Aluminum \| Long
Commodity contract commitments
Commodity contract (in MT/Lbs) \| MT	2,475
Commodity contract \| Futures \| Aluminum \| Short
Commodity contract commitments
Commodity contract (in MT/Lbs) \| MT	2,825
Commodity contract \| Futures \| Copper \| Long
Commodity contract commitments
Commodity contract (in MT/Lbs) \| MT	8,985
Commodity contract \| Futures \| Copper \| Short
Commodity contract commitments
Commodity contract (in MT/Lbs) \| MT	14,144
Commodity contract \| Futures \| Silver \| Short
Commodity contract commitments
Commodity contract (in MT/Lbs) \| lb	343
Firm commitments \| Designated as hedging instrument \| Cost of goods sold
Gains or losses on derivative instruments, statement of income
Amount of gain (loss) recognized in income on related hedged items	$ 2,084,000	1,488,000	120,000
Inventory \| Designated as hedging instrument \| Cost of goods sold
Gains or losses on derivative instruments, statement of income
Amount of gain (loss) recognized in income on related hedged items	$ 1,269,000	$ (3,411,000)	$ (7,437,000)