Derivative Financial Instruments (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) lb Mt	Jun. 30, 2014 USD ($)	Dec. 31, 2014 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	$ (1,803,000)	$ 2,193,000	$ 1,347,000	$ 689,000
Futures | Aluminum | Long
Commodity contract commitments
Commodity contract (in MT/Lbs) | Mt			2,825
Futures | Aluminum | Short
Commodity contract commitments
Commodity contract (in MT/Lbs) | Mt			3,275
Futures | Copper | Long
Commodity contract commitments
Commodity contract (in MT/Lbs) | Mt			8,912
Futures | Copper | Short
Commodity contract commitments
Commodity contract (in MT/Lbs) | Mt			16,041
Futures | Silver | Short
Commodity contract commitments
Commodity contract (in MT/Lbs) | lb			343
Commodity contract | Futures
Fair values of derivative instruments, balance sheet
Commodity futures, asset derivatives (in dollars)	4,472,000		$ 4,472,000		$ 5,312,000
Commodity futures, liability derivatives (in dollars)	3,453,000		3,453,000		1,539,000
Commodity contract | Futures | Other current assets
Fair values of derivative instruments, balance sheet
Fair value of derivatives including required margin deposits	6,400,000		6,400,000		7,600,000
Commodity contract | Futures | Designated as hedging instrument
Gains or losses on derivative instruments, statement of income
Ineffectiveness - amount of gain recognized in income on related hedged items	20,000	160,000	127,000	456,000
Gain (loss) excluded from hedge effectiveness testing	(1,252,000,000)	599,000	18,000	782,000
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	3,075,000	(2,632,000)	(1,238,000)	(1,015,000)
Commodity contract | Futures | Designated as hedging instrument | Other current assets
Fair values of derivative instruments, balance sheet
Commodity futures, asset derivatives (in dollars)	2,508,000		2,508,000		3,180,000
Commodity futures, liability derivatives (in dollars)	1,479,000		1,479,000		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	(326,000)	(2,030,000)	6,670,000	5,926,000
Commodity contract | Futures | Not designated as hedging instrument | Other current assets
Fair values of derivative instruments, balance sheet
Commodity futures, asset derivatives (in dollars)	1,964,000		1,964,000		2,132,000
Commodity futures, liability derivatives (in dollars)	1,974,000		1,974,000		$ 626,000
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	362,000	(653,000)	856,000	331,000
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	$ (2,165,000)	$ 2,846,000	$ 491,000	$ 358,000