Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 2017
Accounting Policies [Abstract]
Principles of Consolidation and Basis of Presentation	Principles of Consolidation and Basis of Presentation. The accompanying unaudited consolidated financial statements include the accounts of our wholly owned subsidiaries and are prepared in accordance with United States generally accepted accounting principles ("GAAP") and the rules and regulations of the Securities and Exchange Commission ("SEC") applicable for interim periods and, therefore, do not include all information and footnotes required by GAAP for complete financial statements. In management's opinion, all adjustments (which include normal recurring adjustments) considered necessary for a fair presentation have been included. The results of operations for our interim periods are not necessarily indicative of the results of operations that may be achieved for the entire 2017 fiscal year. The financial information as of December 31, 2016 is derived from our audited consolidated financial statements and footnotes included in our Annual Report on Form 10-K for the year ended December 31, 2016.
Use of Estimates in the Preparation of Financial Statements	Use of Estimates in the Preparation of Financial Statements. The preparation of financial statements in accordance with GAAP requires the use of estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities known to exist as of the date the financial statements are published and the reported amounts of revenues and expenses during the reporting period. Uncertainties with respect to such estimates and assumptions are inherent in the preparation of our consolidated financial statements; accordingly, it is possible that the actual results could differ from these estimates and assumptions, which could have a material effect on the reported amounts of our consolidated financial position and results of operations.
Inventories	Inventories. Inventories are stated at the lower of cost or market value. On March 31, 2016, we recorded a lower of cost or market inventory write-down of $4.9 million as a result of a decrease in our net realizable value of inventory. The net realizable value reflected commitments as of that date from customers to purchase our inventory at prices that exceeded the Midwest Transaction Price ("Midwest Price"), which reflects the primary aluminum supply/demand dynamics in North America, reduced by an approximate normal profit margin. There were no additional lower of cost or market inventory adjustments during the remainder of 2016 or the first quarter of 2017. Finished products, work-in-process and raw material inventories are stated on the last-in, first-out ("LIFO") basis. At March 31, 2017, the current cost of inventory exceeded its stated LIFO value by $1.9 million. At December 31, 2016 (after inventory write-down adjustments) the stated LIFO value of our inventory represented its net realizable value (less a normal profit margin) and exceeded the current cost of our inventory by $8.5 million. Other inventories are stated on the first-in, first-out basis and consist of operating supplies, which are materials and supplies to be consumed during the production process. Inventory costs consist of material, labor and manufacturing overhead, including depreciation. Abnormal costs, such as idle facility expenses, freight, handling costs and spoilage, are accounted for as current period charges. All of our inventories at March 31, 2017 and December 31, 2016 were included in the Fabricated Products segment (see Note 2 for the components of inventories).
Property, Plant and Equipment - Net	Property, Plant and Equipment – Net. Property, plant and equipment – net is recorded at cost and includes construction in progress (see Note 2). Interest related to the construction of qualifying assets is capitalized as part of the construction costs. The amount of interest expense capitalized as construction in progress was $0.5 million and $0.9 million during the quarters ended March 31, 2017 and March 31, 2016, respectively. Depreciation is computed using the straight-line method at rates based on the estimated useful lives of the various classes of assets. Capital lease assets and leasehold improvements are depreciated on a straight-line basis over the shorter of the estimated useful lives of the assets or the lease term. Depreciation expense is not included in Cost of products sold, excluding depreciation and amortization and other items, but is included in Depreciation and amortization. We classify assets as held for sale only when an asset is being actively marketed and expected to sell within 12 months. Assets held for sale are initially measured at the lesser of the assets' carrying amount and the fair value less costs to sell.
Self Insurance of Employee Health and Workers' Compensation Liabilities	Self Insurance of Workers' Compensation and Employee Healthcare Liabilities. We self-insure the majority of the costs of workers' compensation benefits and employee healthcare benefits and rely on insurance coverage to protect us from large losses on individual claims. Workers' compensation liabilities are based on a combination of estimates for: (i) incurred-but-not-reported claims and (ii) the ultimate expense of incurred claims. Such estimates are based on judgment, using our historical claims data and information and analysis provided by actuarial and claims advisors, our insurance carriers and other professionals. Our undiscounted workers' compensation liabilities were estimated at $26.8 million at both March 31, 2017 and December 31, 2016. However, we account for our workers' compensation accrued liability on a discounted basis, using a discount rate of 2.00% at both March 31, 2017 and December 31, 2016. Accrued liabilities for employee healthcare benefits, which are estimates of unpaid incurred medical and prescription drug costs as provided by our healthcare administrators, were $3.8 million and $3.6 million for the periods ended March 31, 2017 and December 31, 2016, respectively.
Derivative Financial Instruments	Derivative Financial Instruments. Hedging transactions using derivative financial instruments are primarily designed to mitigate our exposure to changes in the market price of aluminum, alloying metals and energy and, to a lesser extent, to mitigate our exposure to changes in foreign currency exchange rates. From time to time, we also enter into hedging arrangements in connection with financing transactions to mitigate financial risks. We do not utilize derivative financial instruments for trading or other speculative purposes. Our derivative activities are initiated within guidelines established by management and approved by our Board of Directors. Hedging transactions are executed centrally on behalf of all of our operations to minimize transaction costs, monitor consolidated net exposures and allow for increased responsiveness to changes in market factors. We recognize derivative instruments as assets or liabilities in our Consolidated Balance Sheets and measure these instruments at fair value by "marking-to-market" all of our hedging positions at each period's end (see Note 8). Because we do not meet the documentation requirements for hedge (deferral) accounting related to aluminum and energy derivatives, unrealized and realized gains and losses associated with these hedges are reflected as a reduction or increase, respectively, in: (i) Unrealized gain on derivative instruments and (ii) Cost of products sold, excluding depreciation and amortization and other items. From time to time, we enter into foreign currency forward contracts to protect the value of anticipated foreign currency expenses associated with cash commitments for equipment purchases. These derivative instruments are designated and qualify for cash flow hedge accounting and are adjusted to current market values each reporting period. Both realized and unrealized periodic gains and losses of derivative instruments designated as cash flow hedges are deferred in Accumulated other comprehensive loss until depreciation on the underlying equipment commences. Upon commencement, realized gains and losses are recorded in Net income as an adjustment to depreciation expense in the period in which depreciation is recognized on the underlying equipment. Additionally, to limit our economic (i.e. cash exposure) to future price increases of alloying metals, such as zinc and/or copper, used as raw materials in our fabrication operations, we enter into forward swap contracts ("Alloy Hedges"). These Alloy Hedges are designated and qualify for cash flow hedge accounting and are adjusted to current market values each reporting period. The effective portion of the fair value on these Alloy Hedges is recorded within Other comprehensive income, net of tax and is reclassified into the Statements of Consolidated Income during the month of settlement to Cost of products sold, excluding depreciation and amortization and other items. Depending on the time to maturity and asset or liability position, the carrying values of cash flow hedges are included in Prepaid expenses and other current assets, Other assets, Other accrued liabilities or Long-term liabilities. We report the effective portion of our cash flow hedges in the same financial statement line item as changes in the fair value of the hedged item.
Fair Value Measurement, Policy [Policy Text Block]	Fair Value Measurements. We apply the fair value hierarchy established by GAAP for the recognition and measurement of certain financial assets and liabilities. An asset or liability's fair value classification within the hierarchy is determined based on the lowest level input that is significant to the fair value measurement. In determining fair value, we utilize valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible and consider counterparty risk in our assessment of fair value. The fair values of financial assets and liabilities are evaluated and measured on a recurring basis. As part of that evaluation process, we review the underlying inputs that are significant to the fair value measurement of financial instruments to determine if a transfer among hierarchy levels is appropriate. We historically have not had significant transfers into or out of each hierarchy level. Financial assets and liabilities that we measure at fair value each period as required by GAAP include our derivative instruments and available for sale securities, consisting of debt investment securities and investments related to our deferred compensation plan (see Note 5). Additionally, we measure at fair value once each year at December 31 the plan assets of the Salaried VEBA (defined in Note 5) and our Canadian defined benefit pension plan. We record our remaining financial assets and liabilities at carrying value. The majority of our non-financial assets and liabilities, which include goodwill, intangible assets, inventories and property, plant and equipment, are not required to be measured at fair value on a recurring basis. However, if certain triggering events occur (or at least annually for goodwill), an evaluation of the affected non-financial asset or liability is required, potentially resulting in an adjustment to the carrying amount of such asset or liability. As of March 31, 2017, following a qualitative review of operations at the Chandler, Arizona (Extrusion) facility, we determined that no adjustment to the carrying value of its goodwill or intangible asset was required. However, if the facility is unable to reduce its costs and/or increase sales volumes in the near term, it is reasonably possible that the carrying values of its related assets, including goodwill, could be reduced within the next 12 months. We concluded that none of our other non-financial assets and liabilities subject to fair value assessments on a non-recurring basis required a material adjustment to the carrying amount of such assets and liabilities for the quarter ended March 31, 2017.
New Accounting Pronouncements	New Accounting Pronouncements. Accounting Standards Update ("ASU") No. 2014-09, Revenue from Contracts with Customers (Topic 606) ("ASU 2014-09"), was issued in May 2014 and requires an entity to recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. In August 2015, ASU 2014-09 was amended by ASU No. 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, which defers the effective date of ASU 2014-09 by one year for all entities and permits early adoption on a limited basis. ASU 2014-09 was subsequently amended by four additional pronouncements: (i) ASU No. 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing; (ii) ASU No. 2016-11, Revenue Recognition (Topic 605) and Derivatives and Hedging (Topic 815): Rescission of SEC Guidance Because of Accounting Standards Updates 2014-09 and 2014-16 Pursuant to Staff Announcements at the March 3, 2016 EITF Meeting; (iii) ASU No. 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients; and (iv) ASU No. 2016-20, Revenue from Contracts with Customers (Topic 606): Technical Corrections and Improvements to Topic 606. We expect to adopt ASU 2014-09, including its subsequent amendments discussed above, using the modified retrospective transition approach for the fiscal year ending December 31, 2018. Based on our initial assessment, we have concluded that revenue from certain contracts will continue to be recognized at a point-in-time, while revenue from other contracts, primarily from our Aero/HS products and Automotive Extrusions, will be required to be recognized over time. We are currently assessing the impact that over-time recognition will have on our consolidated financial statements. ASU No. 2016-02, Leases (Topic 842): Amendments to the Financial Accounting Standards Board Accounting Standards Codification ("ASU 2016-02"), was issued in February 2016. Under ASU 2016-02, lessees will need to recognize a right-of-use asset and a lease liability for virtually all of their leases (other than leases that meet the definition of a short-term lease). For income statement purposes, a dual model was retained, requiring leases to be classified as either operating or finance. Operating leases will result in straight-line expense (similar to current operating leases) while finance leases will result in a front-loaded expense pattern (similar to current capital leases). ASU 2016-02 becomes effective for us in the first quarter of 2019. We are currently assessing the impact and expect the adoption of this ASU in 2019 to have a material impact on our consolidated financial statements. We do not anticipate any material impact on our consolidated financial statements upon the adoption of the following accounting pronouncements: (i) ASU No. 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities; (ii) ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments; and (iii) ASU No. 2017-07, Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. There were no material impacts on our consolidated financial statements resulting from our early adoption in the first quarter of 2017 of the following accounting pronouncements: (i) ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments; (ii) ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash; and (iii) ASU No. 2017-04, Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment.