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Financial Instruments (Narrative) (Details) $ in Thousands, lb in Millions | 12 Months Ended
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Dec. 31, 2018 USD ($) lb | Dec. 31, 2017 USD ($) lb
Derivative [Line Items] |  |
Net asset (liability) | $ (2,720) | $ (558)
Weight of aluminum that hedged future purchase of aluminum to meet fixed-price forward sales contract obligations, lbs | lb | 22.5 | 8.0
Amounts of unrealized after-tax gains on derivative instruments | $ (1,100) |
Aluminum Futures Contracts |  |
Derivative [Line Items] |  |
Notional amount | 25,400 | $ 8,200
Prepaid Expenses and Other Current Assets [Member] | Aluminum Futures Contracts |  |
Derivative [Line Items] |  |
Net asset (liability) | (1,630) |
Cash Flow Hedging [Member] | Foreign Currency Forwwards And Options [Member] |  |
Derivative [Line Items] |  |
Foreign Currency Cash Flow Hedge Derivative at Fair Value, Net | 900 |
Oct-2018 [Member] | Terphane Ltda [Member] |  |
Derivative [Line Items] |  |
Annual Net Costs Mismatch Translation Exposure - Real vs US Dollar | $ 125,000 |

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