Accounting Policies (Policies)	6 Months Ended
Accounting Policies (Policies)	Jun. 30, 2017
Accounting Policies [Abstract]
Basis Of Accounting	The interim consolidated financial statements include the accounts of Allegheny Technologies Incorporated and its subsidiaries. Unless the context requires otherwise, “Allegheny Technologies”, “ATI” and “the Company” refer to Allegheny Technologies Incorporated and its subsidiaries. These unaudited consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles for interim financial information and with the instructions for Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and note disclosures required by U.S. generally accepted accounting principles for complete financial statements. In management’s opinion, all adjustments (which include only normal recurring adjustments) considered necessary for a fair presentation have been included. These unaudited consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto included in the Company’s 2016 Annual Report on Form 10-K. The results of operations for these interim periods are not necessarily indicative of the operating results for any future period. The December 31, 2016 financial information has been derived from the Company’s audited consolidated financial statements.
New Accounting Pronouncements Adopted	New Accounting Pronouncements Adopted In January 2017, the Company early adopted changes issued by the Financial Accounting Standards Board (FASB) to simplify how an entity is required to test goodwill for impairment by eliminating Step 2 from the goodwill impairment test. Step 2 measures a goodwill impairment loss by comparing the implied fair value of a reporting unit’s goodwill with the carrying amount of that goodwill, which is currently required if a reporting unit with goodwill fails a Step 1 test comparing the fair value of the reporting unit to its carrying value including goodwill. Under this new guidance, an entity should perform its annual, or interim, goodwill impairment test using just the Step 1 test of comparing the fair value of a reporting unit with its carrying amount. Any goodwill impairment, representing the amount by which the carrying amount exceeds the reporting unit’s fair value, is determined using this Step 1 test. Any goodwill impairment loss recognized would not exceed the total carrying amount of goodwill allocated to that reporting unit. The adoption of these changes did not have a material impact on the Company’s financial statements. In January 2017, the Company adopted changes issued by the FASB to simplify employee share-based payment accounting. The areas for simplification in this guidance involve several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows, which will be prospectively adopted. The adoption of these changes did not have a material impact on the Company’s financial statements. In January 2017, the Company adopted changes issued by the FASB to simplify the measurement of inventory valuation at the lower of cost or net realizable value. Net realizable value is the estimated selling price in the ordinary course of business, less reasonably predictable costs of completion, disposal and transportation. The new inventory measurement requirements replace the current inventory valuation guidance that requires the use of a lower of cost or market framework. This change in the measurement of inventory does not apply to inventory valued on a LIFO basis, which is the accounting basis used for most of the Company’s inventory. The adoption of these changes did not have a material impact on the Company’s financial statements.
New Accounting Pronouncements Not Yet Adopted	Pending Accounting Pronouncement In March 2017, the FASB issued changes to the accounting for defined benefit pension and other postretirement benefit expenses. This new guidance requires the disaggregation of the service cost component from the other components of net benefit cost. The service cost component of net benefit cost is to be reported in the same line item on the consolidated statement of operations as other compensation costs arising from services rendered by the pertinent employees, while the other components of net benefit cost are to be presented in the consolidated statement of operations separately, outside a subtotal of operating income. The amendments also provide explicit guidance to allow only the service cost component of net benefit cost to be eligible for capitalization. This new guidance is effective for the Company’s 2018 fiscal year, with the adoption of the change in presentation of net benefit cost in the consolidated statement of operations to be applied retrospectively, and the change in capitalization for only service cost applied prospectively. The guidance allows a practical expedient that permits the use of the amounts disclosed in the retirement benefits footnote for the prior comparative periods as the estimation basis for applying the retrospective presentation requirements. The Company will adopt this new guidance in the first quarter of fiscal year 2018 using this practical expedient. The Company expects such adoption to have a material impact to reported operating income due to the change in presentation of non-service cost expense components. For example, applying the practical expedient to fiscal year 2016 results, operating income for 2016 would be $70.6 million higher, with the reclassification of this amount representing the other components of net benefit cost to a newly-created non-operating retirement benefit expense category, with no net impact to the reported 2016 loss before income taxes. The Company expects to have a one-time, unfavorable impact to pre-tax reported results in the first quarter of 2018 upon adoption due to the change limiting only the service cost component of net benefit cost to be capitalizable into inventory. Using expected fiscal year 2017 defined benefit retirement expense and current inventory levels, this impact in the first quarter of 2018 is projected to be approximately $5 million, pre-tax. In February 2016, the FASB issued new guidance on the accounting for leases. This new guidance will require that a lessee recognize assets and liabilities on the balance sheet for all leases with a lease term of more than twelve months, with the result being the recognition of a right of use asset and a lease liability. The new lease accounting requirements are effective for the Company’s 2019 fiscal year with a modified retrospective transition approach required, with early adoption permitted. The Company is currently evaluating the impact of the new guidance on its consolidated financial statements. In May 2014, the FASB issued changes to revenue recognition with customers, which is required to be adopted by the Company in fiscal year 2018. This update provides a five-step analysis of transactions to determine when and how revenue is recognized, along with expanded disclosure requirements. An entity should 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. The Company plans to adopt this accounting standard update using the modified retrospective method, with the cumulative effect of initially applying this update recognized in the first reporting period of 2018. Although the Company is currently evaluating the impact of this standard on individual customer contracts, the Company has evaluated the impact of this standard on the broad categories of its customer contracts, and anticipates the adoption of this guidance will not have a material impact on the consolidated statement of operations but does expect an impact to the consolidated balance sheet for reclassifications to contract assets and liabilities, the magnitude of which is still being determined. The Company also expects an increase to financial statement footnote disclosures regarding revenues, contract assets and contract liabilities as a result of this accounting standard update.
Inventory	Inventories are stated at the lower of cost (last-in, first-out (LIFO), first-in, first-out (FIFO), and average cost methods) or market, less progress payments. Most of the Company’s inventory is valued utilizing the LIFO costing methodology. Inventory of the Company’s non-U.S. operations is valued using average cost or FIFO methods. Due to deflationary impacts primarily related to raw materials, the carrying value of the Company’s inventory as valued on LIFO exceeds current replacement cost, and based on a lower of cost or market value analysis, a net realizable value (NRV) inventory reserve is required.
Derivatives	As part of its risk management strategy, the Company, from time-to-time, utilizes derivative financial instruments to manage its exposure to changes in raw material prices, energy costs, foreign currencies, and interest rates. In accordance with applicable accounting standards, the Company accounts for most of these contracts as hedges. In general, hedge effectiveness is determined by examining the relationship between offsetting changes in fair value or cash flows attributable to the item being hedged, and the financial instrument being used for the hedge. Effectiveness is measured utilizing regression analysis and other techniques to determine whether the change in the fair market value or cash flows of the derivative exceeds the change in fair value or cash flow of the hedged item. Calculated ineffectiveness, if any, is immediately recognized in the consolidated statements of operations. The Company sometimes uses futures and swap contracts to manage exposure to changes in prices for forecasted purchases of raw materials, such as nickel, and natural gas. Under these contracts, which are generally accounted for as cash flow hedges, the price of the item being hedged is fixed at the time that the contract is entered into and the Company is obligated to make or receive a payment equal to the net change between this fixed price and the market price at the date the contract matures. The majority of ATI’s products are sold utilizing raw material surcharges and index mechanisms. However, as of June 30, 2017, the Company had entered into financial hedging arrangements, primarily at the request of its customers, related to firm orders, for an aggregate notional amount of approximately 27 million pounds of nickel with hedge dates through 2021. The aggregate notional amount hedged is approximately 35% of a single year’s estimated nickel raw material purchase requirements. At June 30, 2017, the outstanding financial derivatives used to hedge the Company’s exposure to energy cost volatility included natural gas cost hedges. In the first six months of 2016, due to changes in expected operating levels within Flat Rolled Products segment operations, the Company concluded that additional portions of these natural gas cash flow hedges for 2016 and the first quarter of 2017 were ineffective based on forecast changes in underlying natural gas usage. The Company recognized $0.2 million and $1.3 million of pre-tax losses for the three and six months ended June 30, 2016, respectively, for natural gas cash flow hedge ineffectiveness, which is reported in selling and administrative expenses on the consolidated statement of operations. At June 30, 2017 the Company hedged approximately 70% of the Company’s forecasted domestic requirements for natural gas for the remainder of 2017, approximately 35% for 2018, and approximately 25% for 2019. While the majority of the Company’s direct export sales are transacted in U.S. dollars, foreign currency exchange contracts are used, from time-to-time, to limit transactional exposure to changes in currency exchange rates for those transactions denominated in a non-U.S. currency. The Company sometimes purchases foreign currency forward contracts that permit it to sell specified amounts of foreign currencies expected to be received from its export sales for pre-established U.S. dollar amounts at specified dates. The forward contracts are denominated in the same foreign currencies in which export sales are denominated. These contracts are designated as hedges of the variability in cash flows of a portion of the forecasted future export sales transactions which otherwise would expose the Company to foreign currency risk, primarily euros. In addition, the Company may also designate cash balances held in foreign currencies as hedges of forecasted foreign currency transactions. In 2015, the Company net settled substantially all of its foreign currency forward contracts designated as cash flow hedges with 2016 and 2017 maturity dates. The portion of the deferred gains on these settled cash flow hedges determined to be effective is currently recognized in accumulated other comprehensive income and is reclassified to earnings when the underlying transactions occur. As of June 30, 2017, the Company held 21.6 million euro notional value of foreign currency forward contracts designated as fair value hedges with maturity dates through 2017. The Company recorded $1.9 million and $2.2 million of charges in the three and six months ended June 30, 2017, respectively, and a $3.1 million benefit and a $2.5 million charge in the three and six months ended June 30, 2016, respectively, in costs of sales on the consolidated statement of operations for maturities and mark-to-market changes on these fair value hedges. The Company may enter into derivative interest rate contracts to maintain a reasonable balance between fixed- and floating-rate debt. There were no unsettled derivative financial instruments related to debt balances for the periods presented. There are no credit risk-related contingent features in the Company’s derivative contracts, and the contracts contained no provisions under which the Company has posted, or would be required to post, collateral. The counterparties to the Company’s derivative contracts are substantial and creditworthy commercial banks that are recognized market makers. The Company controls its credit exposure by diversifying across multiple counterparties and by monitoring credit ratings and credit default swap spreads of its counterparties. The Company also enters into master netting agreements with counterparties when possible. For derivative financial instruments that are designated as cash flow hedges, the effective portion of the gain or loss on the derivative is reported as a component of other comprehensive income (OCI) and reclassified into earnings in the same period or periods during which the hedged item affects earnings. Gains and losses on the derivative representing either hedge ineffectiveness or hedge components excluded from the assessment of effectiveness are recognized in current period results. For derivative financial instruments that are designated as fair value hedges, changes in the fair value of these derivatives are recognized in current period results and are reported as changes within accrued liabilities and other on the consolidated statements of cash flows. The Company did not use net investment hedges for the periods presented. The effects of derivative instruments in the tables below are presented net of related income taxes, excluding any impacts of changes to income tax valuation allowances effecting results of operations or other comprehensive income, when applicable (see Note 12 for further explanation).
Retirement Benefits	The Company has defined contribution retirement plans or defined benefit pension plans covering substantially all employees. Company contributions to defined contribution retirement plans are generally based on a percentage of eligible pay or based on hours worked. Benefits under the defined benefit pension plans are generally based on years of service and/or final average pay. The Company funds the U.S. pension plans in accordance with the Employee Retirement Income Security Act of 1974, as amended, and the Internal Revenue Code. The Company also sponsors several postretirement plans covering certain collectively-bargained salaried and hourly employees. The plans provide health care and life insurance benefits for eligible retirees. In most retiree health care plans, Company contributions towards premiums are capped based on the cost as of a certain date, thereby creating a defined contribution.
Commitments And Contingencies	Environmental liabilities are recorded when the Company’s liability is probable and the costs are reasonably estimable. In many cases, however, the Company is not able to determine whether it is liable or, if liability is probable, to reasonably estimate the loss or range of loss. Estimates of the Company’s liability remain subject to additional uncertainties, including the nature and extent of site contamination, available remediation alternatives, the extent of corrective actions that may be required, and the number, participation, and financial condition of other potentially responsible parties (PRPs). The Company adjusts its accruals to reflect new information as appropriate. Future adjustments could have a material adverse effect on the Company’s consolidated results of operations in a given period, but the Company cannot reliably predict the amounts of such future adjustments.

Disclosure of accounting policy for pension and other postretirement benefit plans. This accounting policy may address (1) the types of plans sponsored by the entity, and the benefits provided by each plan (2) groups that participate in (or are covered by) each plan (3) how plan assets, liabilities and expenses are measured, including the use of any actuaries and (4) significant assumptions used by the entity to value plan assets and liabilities and how such assumptions are derived.