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Derivative Financial Instruments (Details)	3 Months Ended		6 Months Ended
Derivative Financial Instruments (Details)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) Mt	Jun. 30, 2015 USD ($)	Dec. 31, 2015 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	$ 333,000	$ (1,803,000)	$ (611,000)	$ 1,347,000
Commodity contract \| Futures
Fair values of derivative instruments, balance sheet
Commodity futures, asset derivatives (in dollars)	1,678,000		1,678,000		$ 1,765,000
Commodity futures, liability derivatives (in dollars)	4,838,000		4,838,000		3,925,000
Commodity contract \| Futures \| Other current assets
Fair values of derivative instruments, balance sheet
Fair value of derivatives including required margin deposits	922,000		922,000		3,400,000
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	(47,000)	20,000	(91,000)	127,000
Gain (loss) excluded from hedge effectiveness testing	(97,000)	1,252,000	(65,000)	(18,000)
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	(477,000)	3,075,000	455,000	(1,238,000)
Commodity contract \| Futures \| Designated as hedging instrument \| Other current assets
Fair values of derivative instruments, balance sheet
Commodity futures, asset derivatives (in dollars)	477,000		477,000		857,000
Commodity futures, liability derivatives (in dollars)	2,009,000		2,009,000		2,860,000
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	1,116,000	(326,000)	244,000	6,670,000
Commodity contract \| Futures \| Not designated as hedging instrument \| Other current assets
Fair values of derivative instruments, balance sheet
Commodity futures, asset derivatives (in dollars)	1,201,000		1,201,000		908,000
Commodity futures, liability derivatives (in dollars)	2,829,000		$ 2,829,000		$ 1,065,000
Commodity contract \| Futures \| Aluminum \| Long
Commodity contract commitments
Commodity contract (in MT/Lbs) \| Mt			1,725
Commodity contract \| Futures \| Aluminum \| Short
Commodity contract commitments
Commodity contract (in MT/Lbs) \| Mt			1,095
Commodity contract \| Futures \| Copper \| Long
Commodity contract commitments
Commodity contract (in MT/Lbs) \| Mt			15,809
Commodity contract \| Futures \| Copper \| Short
Commodity contract commitments
Commodity contract (in MT/Lbs) \| Mt			16,375
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	(208,000)	362,000	$ (1,430,000)	856,000
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	$ 541,000	$ (2,165,000)	$ 819,000	$ 491,000