Derivative Financial Instruments (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) lb Mt	Sep. 30, 2014 USD ($)	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	$ 1,462,000	$ (3,379,000)	$ 2,809,000	$ (2,690,000)
Futures | Aluminum | Long
Commodity contract commitments
Commodity contract (in MT/Lbs) | Mt			2,350
Futures | Aluminum | Short
Commodity contract commitments
Commodity contract (in MT/Lbs) | Mt			2,275
Futures | Copper | Long
Commodity contract commitments
Commodity contract (in MT/Lbs) | Mt			7,602
Futures | Copper | Short
Commodity contract commitments
Commodity contract (in MT/Lbs) | Mt			13,860
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)	2,249,000		$ 2,249,000		$ 5,312,000
Commodity futures, liability derivatives (in dollars)	3,429,000		3,429,000		1,539,000
Commodity contract | Futures | Other current assets
Fair values of derivative instruments, balance sheet
Fair value of derivatives including required margin deposits	1,800,000		1,800,000		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	(191,000)	(229,000)	(64,000)	227,000
Gain (loss) excluded from hedge effectiveness testing	(1,200,000)	1,200,000	(1,200,000)	439,000
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	(2,825,000)	4,371,000	(4,063,000)	3,356,000
Commodity contract | Futures | Designated as hedging instrument | Other current assets
Fair values of derivative instruments, balance sheet
Commodity futures, asset derivatives (in dollars)	614,000		614,000		3,180,000
Commodity futures, liability derivatives (in dollars)	2,410,000		2,410,000		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	6,707,000	2,672,000	13,377,000	8,598,000
Commodity contract | Futures | Not designated as hedging instrument | Other current assets
Fair values of derivative instruments, balance sheet
Commodity futures, asset derivatives (in dollars)	1,635,000		1,635,000		2,132,000
Commodity futures, liability derivatives (in dollars)	1,019,000		1,019,000		$ 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	662,000	784,000	1,518,000	1,115,000
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	$ 800,000	$ (4,163,000)	$ 1,291,000	$ (3,805,000)