Summary of Significant Accounting Policies - Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the accounts of Allegheny Technologies Incorporated and its subsidiaries, including the Chinese joint venture known as Shanghai STAL Precision Stainless Steel Company Limited (STAL), in which the Company has a 60% interest. The remaining 40% interest in STAL is owned by China Baowu Steel Group Corporation Limited, a state authorized investment company whose equity securities are publicly traded in the People’s Republic of China. The financial results of STAL are consolidated into the Company’s operating results and financial position, with the 40% interest of our minority partner recognized in the consolidated statement of operations as net income attributable to noncontrolling interests and as equity attributable to the noncontrolling interest within total stockholders’ equity. Investments in which the Company exercises significant influence, but which it does not control (generally a 20% to 50% ownership interest), including ATI’s 50% interest in the industrial titanium joint venture known as Uniti LLC (“Uniti”), are accounted for under the equity method of accounting. Accounts receivable from Uniti were $1.5 million and $0.5 million at December 31, 2016 and 2015, respectively. Significant intercompany accounts and transactions have been eliminated. Unless the context requires otherwise, “Allegheny Technologies,” “ATI” and the “Company” refer to Allegheny Technologies Incorporated and its subsidiaries.
Use of Estimates	Risks and Uncertainties and Use of Estimates The preparation of consolidated financial statements in conformity with United States generally accepted accounting principles requires management to make estimates and assumptions that affect reported amounts of assets and liabilities at the date of the financial statements, as well as the reported amounts of income and expenses during the reporting period. Actual results could differ from those estimates. Management believes that the estimates are reasonable. Certain prior year amounts have been reclassified in order to conform with the 2016 presentation (see Note 5).
Concentration Risks	The Company markets its products to a diverse customer base, principally throughout the United States. No single customer accounted for more than 10% of sales for any year presented. The principal end markets for the ATI’s products are customers in the aerospace & defense, oil & gas, electrical energy, automotive, construction and mining, food equipment and appliances, and medical markets.
Cash Equivalents and Investments	Cash Equivalents and Investments Cash equivalents are highly liquid investments valued at cost, which approximates fair value, acquired with an original maturity of three months or less.
Accounts Receivable	Accounts Receivable Accounts receivable are presented net of a reserve for doubtful accounts of $7.3 million and $4.5 million at December 31, 2016 and 2015, respectively. Trade credit is extended based upon evaluations of each customer’s ability to perform its obligations, which are updated periodically. Accounts receivable reserves are determined based upon an aging of accounts and a review for collectability of specific accounts.
Inventories	Inventories 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. Costs include direct material, direct labor and applicable manufacturing and engineering overhead, and other direct costs. 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. The Company evaluates product lines on a quarterly basis to identify inventory carrying values that exceed estimated net realizable value. In applying the lower of cost or market principle, market means current replacement cost, subject to a ceiling (market value shall not exceed net realizable value) and a floor (market shall not be less than net realizable value reduced by an allowance for a normal profit margin). The calculation of a resulting reserve, if any, is recognized as an expense in the period that the need for the reserve is identified. However, in cases where inventory at FIFO cost is lower than the LIFO carrying value, a write-down of the inventory to market may be required, subject to the ceiling and floor. It is the Company’s general policy to write-down to scrap value any inventory that is identified as slow-moving or aged more than twelve months, subject to sales, backlog and anticipated orders considerations. In some instances this aging criterion is up to twenty-four months.
Long-Lived Assets	Long-Lived Assets Property, plant and equipment are recorded at cost, including capitalized interest, and includes long-lived assets acquired under capital leases. The principal method of depreciation adopted for all property placed into service after July 1, 1996 is the straight-line method. For buildings and equipment acquired prior to July 1, 1996, depreciation is computed using a combination of accelerated and straight-line methods. Property, plant and equipment associated with the Company’s Rowley titanium sponge production facility in the High Performance Materials & Components segment (prior to its indefinite idling in August 2016- see Note 17 for further explanation), and the Hot Rolling and Processing Facility (HRPF) in the Flat Rolled Products segment, are being depreciated utilizing the units of production method of depreciation, which the Company believes provides a better matching of costs and revenues. The Company periodically reviews estimates of useful life and production capacity assigned to new and in service assets. Significant enhancements, including major maintenance activities that extend the lives of property and equipment, are capitalized. Costs related to repairs and maintenance are charged to expense in the period incurred. The cost and related accumulated depreciation of property and equipment retired or disposed of are removed from the accounts and any related gains or losses are included in income. The Company monitors the recoverability of the carrying value of its long-lived assets. An impairment charge is recognized when an indicator of impairment occurs and the expected net undiscounted future cash flows from an asset’s use (including any proceeds from disposition) are less than the asset’s carrying value and the asset’s carrying value exceeds its fair value. Assets to be disposed of by sale are stated at the lower of their fair values or carrying amounts and depreciation is no longer recognized.
Goodwill	Goodwill Goodwill is reviewed annually for impairment or more frequently if impairment indicators arise. The review for goodwill impairment requires a comparison of the fair value of each reporting unit that has goodwill associated with its operations with its carrying amount, including goodwill. If this comparison reflects impairment, then the loss would be measured as the excess of recorded goodwill over its implied fair value. Implied fair value is the excess of the fair value of a reporting unit over the amounts assigned to its assets and liabilities. Generally accepted accounting standards provide the option to qualitatively assess goodwill for impairment before completing a quantitative assessment. Under the qualitative approach, if, after assessing the totality of events or circumstances, including both macroeconomic, industry and market factors, and entity-specific factors, the Company determines it is likely (more likely than not) that the fair value of a reporting unit is greater than its carrying amount, then the quantitative impairment analysis is not required. The quantitative assessment may be performed each year for a reporting unit at the Company’s option without first performing a qualitative assessment. The Company’s quantitative assessment of goodwill for possible impairment includes estimating the fair market value of a reporting unit which has goodwill associated with its operations using discounted cash flow and multiples of cash earnings valuation techniques, plus valuation comparisons to recent public sale transactions of similar businesses, if any. These impairment assessments and valuation methods require the Company to make estimates and assumptions regarding future operating results, cash flows, changes in working capital and capital expenditures, selling prices, profitability, and the cost of capital. Many of these assumptions are determined by reference to market participants identified by the Company. Although management believes that the estimates and assumptions used were reasonable, actual results could differ from those estimates and assumptions. Other events and changes in circumstances may also require goodwill to be tested for impairment between annual measurement dates. While a decline in stock price and market capitalization is not specifically cited as a goodwill impairment indicator, a company’s stock price and market capitalization should be considered in determining whether it is more likely than not that the fair value of a reporting unit is less that its carrying value. Additionally, a significant decline in a company’s stock price may suggest that an adverse change in the business climate may have caused the fair value of one or more reporting units to fall below carrying value. A sustained decline in market capitalization below book value may be determined to require an interim goodwill impairment review.
Environmental	Environmental Costs that mitigate or prevent future environmental contamination or extend the life, increase the capacity or improve the safety or efficiency of property utilized in current operations are capitalized. Other costs that relate to current operations or an existing condition caused by past operations are expensed. Environmental liabilities are recorded when the Company’s liability is probable and the costs are reasonably estimable, but generally not later than the completion of the feasibility study or the Company’s recommendation of a remedy or commitment to an appropriate plan of action. The accruals are reviewed periodically and, as investigations and remediations proceed, adjustments of the accruals are made to reflect new information as appropriate. Accruals for losses from environmental remediation obligations do not take into account the effects of inflation, and anticipated expenditures are not discounted to their present value. The accruals are not reduced by possible recoveries from insurance carriers or other third parties, but do reflect allocations among potentially responsible parties (PRPs) at Federal Superfund sites or similar state-managed sites after an assessment is made of the likelihood that such parties will fulfill their obligations at such sites and after appropriate cost-sharing or other agreements are entered. The measurement of environmental liabilities by the Company is based on currently available facts, present laws and regulations, and current technology. Such estimates take into consideration the Company’s prior experience in site investigation and remediation, the data concerning cleanup costs available from other companies and regulatory authorities, and the professional judgment of the Company’s environmental experts in consultation with outside environmental specialists, when necessary.
Foreign Currency Translation	Foreign Currency Translation Assets and liabilities of international operations are translated into U.S. dollars using year-end exchange rates, while revenues and expenses are translated at average exchange rates during the period. The resulting net translation adjustments are recorded as a component of accumulated other comprehensive income (loss) in stockholders’ equity.
Sales Recognition	Sales Recognition Sales are recognized when title passes or as services are rendered.
Research and Development	Research and Development Company funded research and development costs from continuing operations were $14.7 million in 2016, $14.2 million in 2015, and $17.4 million in 2014 and were expensed as incurred. Customer funded research and development costs were $2.2 million in 2016, $1.5 million in 2015, and $2.7 million in 2014.
Stock-based Compensation	Stock-based Compensation The Company accounts for stock-based compensation transactions, such as nonvested stock and performance equity awards, using fair value. Compensation expense for an award is estimated at the date of grant and is recognized over the requisite service period. Compensation expense is adjusted for equity awards that do not vest because service or performance conditions are not satisfied. However, compensation expense already recognized is not adjusted if market conditions are not met, such as the Company’s total shareholder return performance relative to a peer group under certain of the Company’s performance equity awards.
Income Taxes	Income Taxes The provision for, or benefit from, income taxes includes deferred taxes resulting from temporary differences in income for financial and tax purposes using the liability method. Such temporary differences result primarily from differences in the carrying value of assets and liabilities. Future realization of deferred income tax assets requires sufficient taxable income within the carryback and/or carryforward period available under tax law. The Company evaluates on a quarterly basis whether, based on all available evidence, it is probable that the deferred income tax assets are realizable. Valuation allowances are established when it is estimated that it is more likely than not that the tax benefit of the deferred tax asset will not be realized. The evaluation includes the consideration of all available evidence, both positive and negative, regarding historical operating results including recent years with reported losses, the estimated timing of future reversals of existing taxable temporary differences, estimated future taxable income exclusive of reversing temporary differences and carryforwards, and potential tax planning strategies which may be employed to prevent an operating loss or tax credit carryforward from expiring unused. It is the Company’s policy to classify interest and penalties recognized on underpayment of income taxes as income tax expense.
Net Income Per Common Share	Net Income Per Common Share Basic and diluted net income per share are calculated by dividing the net income available to common stockholders by the weighted average number of common shares outstanding during the year. Diluted amounts assume the issuance of common stock for all potentially dilutive share equivalents outstanding. The calculations of all diluted income/loss per share figures for a period exclude the potentially dilutive effect of dilutive share equivalents if there is a net loss from continuing operations since the inclusion in the calculation of additional shares in the net loss from continuing operations per share would result in a lower per share loss and therefore be anti-dilutive.
New Accounting Pronouncements Adopted	New Accounting Pronouncements Adopted In December 2016, the Company adopted changes issued by the Financial Accounting Standards Board (FASB) to fair value measurement disclosures for investments that calculate net asset value per share. This update removes the requirement to categorize within the fair value hierarchy all investments for which fair value is measured using the net asset value per share as a practical expedient. This update also removes the requirement to make certain disclosures for all investments that are eligible to be measured at fair value using the net asset value per share as a practical expedient. The adoption of these changes impacts the fair value disclosures presented in Note 12 and requires retrospective application by removing investments measured using net asset value as a practical expedient from the fair value hierarchy in all periods presented.
New Accounting Pronouncements Not yet Adopted	Pending Accounting Pronouncements In January 2017, the FASB issued changes to accounting standards 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. This new guidance is effective for fiscal years beginning after December 15, 2020, with early adoption permitted. The Company will early adopt this new guidance in 2017 and does not anticipate that it will have a material impact on its consolidated financial statements. In March 2016, the FASB issued new guidance 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 consolidated statement of cash flows. This new guidance is effective for the Company’s 2017 fiscal year. The adoption of this new guidance will not have a material impact on the Company’s consolidated financial statements. 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 July 2015, the FASB issued changes 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 are effective for the Company’s 2017 fiscal year, and will 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 is not expected to have a material impact on the Company’s 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.
Asset Retirement Obligations	Asset Retirement Obligations The Company maintains reserves where a legal obligation exists to perform an asset retirement activity and the fair value of the liability can be reasonably estimated. These asset retirement obligations (AROs) include liabilities where the timing and (or) method of settlement may be conditional on a future event, that may or may not be within the control of the entity. At December 31, 2016, the Company had recognized AROs of $23.3 million related to landfill closures, decommissioning costs, facility leases and conditional AROs associated with manufacturing activities using what may be characterized as potentially hazardous materials. Estimates of AROs are evaluated annually in the fourth quarter, or more frequently if material new information becomes known. Accounting for asset retirement obligations requires significant estimation and in certain cases, the Company has determined that an ARO exists, but the amount of the obligation is not reasonably estimable. The Company may determine that additional AROs are required to be recognized as new information becomes available.
Derivative Financial Instruments and Hedging	Derivative Financial Instruments and Hedging 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 on the statement 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 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 December 31, 2016, 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 28 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 December 31, 2016, the outstanding financial derivatives used to hedge the Company’s exposure to energy cost volatility included natural gas hedges. During the fiscal years ended December 31, 2016 and 2015, due to changes in expected operating levels, the Company concluded that portions of these natural gas cash flow hedges for 2016 and the first quarter 2017 were ineffective based on forecast changes in underlying natural gas usage. The Company recognized $1.3 million and $3.3 million of pre-tax losses for the ineffective portion of these cash flow hedges for the years ended December 31, 2016 and 2015, respectively, which is reported in selling and administrative expenses on the consolidated statement of operations. Approximately 90% of the Company’s annual forecasted domestic requirements for natural gas for 2017 and approximately 25% for 2018 are hedged. 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. During the fiscal year ended December 31, 2015, the Company net settled 211.9 million euro notional value of foreign currency forward contracts designated as cash flow hedges with 2016 and 2017 maturity dates, receiving cash proceeds of $56.5 million which is reported in cash provided by operating activities on the consolidated statement of cash flows. In the fourth quarter 2015, due to management actions in the Flat Rolled Products segment to de-emphasize commodity stainless steel sheet products in 2016, the Company concluded that a portion of these settled euro cash flow hedges for 2016 were ineffective based on forecast changes for euro-denominated sales. The Company recognized a $14.3 million pre-tax gain for the ineffective portion of these cash flow hedges, which is reported in selling and administrative expenses on the consolidated statement of operations for the year ended December 31, 2015. 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 will be reclassified to earnings when the underlying transactions occur. In 2015, the Company entered into 244.7 million euro notional value of foreign currency forward contracts designated as fair value hedges with 2015, 2016 and 2017 maturity dates to replace a portion of the settled euro cash flow hedges, of which 43.2 million and 139.2 million euro notional values were outstanding as of December 31, 2016 and 2015, respectively. The Company recorded benefits of $1.0 million and $9.0 million in costs of sales on the consolidated statement of operations during the fiscal years ended December 31, 2016 and 2015, respectively, 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 were 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. 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 affecting results of operations or other comprehensive income (see Note 13 for further explanation).
Pension Plans and Other Postretirement Benefits	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. defined benefit pension plans in accordance with the Employee Retirement Income Security Act of 1974, as amended, and the Internal Revenue Code (“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.