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Derivative Financial Instruments (Narrative) (Details)
$ in Thousands, lb in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2017
USD ($)
lb
Dec. 31, 2016
USD ($)
lb
Derivative [Line Items]    
Amounts of unrealized after tax gains (losses) to be Reclassified within Twelve Months $ (200)  
Aluminum Futures Contracts    
Derivative [Line Items]    
Notional Amount $ 8,100 $ 8,000
Weight of aluminum that hedged future purchase of aluminum to meet fixed - price forward sales contract obligations, lbs | lb 8.9 9.6
Net asset (liability), Fair Value $ 241 $ 271
Accrued Expenses [Member] | Derivatives Designated As Hedging Instruments [Member] | Aluminum Futures Contracts    
Derivative [Line Items]    
Derivative Asset, Fair Value, Gross Asset 411 308
Derivative Liability, Fair Value, Gross Liability $ 170 $ 37