Accounting Policies	9 Months Ended
Sep. 30, 2018
Accounting Policies [Abstract]
Accounting Policies	Accounting Policies 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 2017 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, 2017 financial information has been derived from the Company’s audited consolidated financial statements. New Accounting Pronouncements Adopted In January 2018, the Company adopted changes issued by the Financial Accounting Standards Board (FASB) related to revenue recognition with customers. See Note 2 for further explanation related to this adoption, including all newly expanded disclosure requirements. In January 2018, the Company adopted changes issued by the FASB related to 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. With this adoption, the change in presentation of net benefit cost in the consolidated statement of operations was applied retrospectively, and the change in capitalization for only service cost was applied prospectively. The Company adopted this new guidance using the 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. This required accounting change did have a material impact to previously-reported operating income (loss) in the consolidated statement of operations due to the change in presentation of non-service cost expense components. For the third quarter and first nine months of 2017, applying the practical expedient, operating results were higher by $13.6 million and $40.9 million, respectively, with the reclassification of this amount representing the other components of net benefit cost to a newly-created nonoperating retirement benefit expense category. There was no net impact to the reported results before income taxes as a result of this accounting change. This change in presentation of net benefit cost did not affect ATI’s measure of segment operating profit; all defined benefit pension and other postretirement benefit expense attributable to business segment operations remains a component of business segment financial performance. The Company did have a one-time, unfavorable impact of $5.4 million to pre-tax reported results in the first quarter of 2018 upon adoption, primarily affecting the Flat Rolled Products business segment, due to the change limiting only the service cost component of net benefit cost to be capitalizable into inventory. In January 2018, the Company early adopted changes issued by the FASB related to changes to its accounting guidance for derivatives and hedging, which changes both the designation and measurement guidance for qualifying hedging relationships and the presentation of hedge results. Some changes resulting from this new guidance include the elimination of the concept of recognizing periodic hedge ineffectiveness for cash flow hedges, changes to the recognition and presentation of changes in the fair value of the hedging instrument, enhancement of the ability to use the critical-terms-match method for the cash flow hedge of groups of forecasted transactions when the timing of the hedged transactions does not perfectly match the hedging instrument’s maturity date, and the addition of new disclosure requirements and amendments to existing ones. The Company applied this new guidance to hedging relationships existing on January 1, 2018, the date of adoption. The adoption of these changes did not have a material impact on the Company's financial statements, and disclosures in Note 7 reflect the requirements of this adoption. In August 2018, the FASB issued new accounting guidance for implementation costs incurred in a cloud computing arrangement that is a service contract. This guidance specifies which implementation costs can be capitalized in a hosting arrangement that is a service contract. The guidance aligns the accounting for implementation costs for hosting arrangements, regardless of whether they convey a license to the hosted software. This guidance is required to be adopted either retrospectively or prospectively by the Company beginning in fiscal year 2020 with early adoption permitted, including adoption in any interim period. The Company early adopted this guidance in the third quarter of 2018 on a retrospective basis, which did not have a material impact on the Company’s consolidated financial statements. Pending Accounting Pronouncements In February 2016, the FASB issued new accounting guidance 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 ATI’s 2019 fiscal year. In July 2018, the FASB added an additional adoption method, which ATI will use to initially apply the new standard at the adoption date, January 1, 2019. This adoption method recognizes any impact to prior years’ results as a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. The Company is currently in the process of evaluating its existing lease portfolio, including accumulating all of the necessary information required to properly account for the leases under the new standard. In addition, ATI is implementing a company-wide lease management system to assist in the accounting and is evaluating additional changes to its processes and internal controls to ensure the standard’s reporting and disclosure requirements are met. The Company is currently evaluating the impact of the new guidance on its consolidated financial statements. In August 2018, the FASB issued new disclosure guidance on fair value measurement. This new guidance modifies the disclosure requirements on fair value measurements, including removal and modifications of various current disclosures as well as some additional disclosure requirements for Level 3 fair value measurements. Some of these disclosure changes must be applied prospectively while others retrospectively depending on requirement. This guidance is required to be adopted by the Company beginning in fiscal year 2020 with early adoption permitted. The Company does not plan to early adopt this guidance. The adoption of these changes is not expected to have an impact on the Company’s consolidated financial statements other than disclosures. In August 2018, the FASB issued new disclosure guidance for defined benefit plans, which modified certain required disclosures. This guidance is required to be adopted retrospectively by the Company beginning in fiscal year 2021, with early adoption permitted. The Company plans to early adopt this guidance in the fourth quarter of 2018, which is not expected to have an impact on the Company’s consolidated financial statements other than disclosures.

Accounting Policies

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 2017 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, 2017 financial information has been derived from the Company’s audited consolidated financial statements.

New Accounting Pronouncements Adopted

In January 2018, the Company adopted changes issued by the Financial Accounting Standards Board (FASB) related to revenue recognition with customers. See Note 2 for further explanation related to this adoption, including all newly expanded disclosure requirements.

In January 2018, the Company adopted changes issued by the FASB related to 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. With this adoption, the change in presentation of net benefit cost in the consolidated statement of operations was applied retrospectively, and the change in capitalization for only service cost was applied prospectively. The Company adopted this new guidance using the 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. This required accounting change did have a material impact to previously-reported operating income (loss) in the consolidated statement of operations due to the change in presentation of non-service cost expense components. For the third quarter and first nine months of 2017, applying the practical expedient, operating results were higher by $13.6 million and $40.9 million, respectively, with the reclassification of this amount representing the other components of net benefit cost to a newly-created nonoperating retirement benefit expense category. There was no net impact to the reported results before income taxes as a result of this accounting change. This change in presentation of net benefit cost did not affect ATI’s measure of segment operating profit; all defined benefit pension and other postretirement benefit expense attributable to business segment operations remains a component of business segment financial performance. The Company did have a one-time, unfavorable impact of $5.4 million to pre-tax reported results in the first quarter of 2018 upon adoption, primarily affecting the Flat Rolled Products business segment, due to the change limiting only the service cost component of net benefit cost to be capitalizable into inventory.

In January 2018, the Company early adopted changes issued by the FASB related to changes to its accounting guidance for derivatives and hedging, which changes both the designation and measurement guidance for qualifying hedging relationships and the presentation of hedge results. Some changes resulting from this new guidance include the elimination of the concept of recognizing periodic hedge ineffectiveness for cash flow hedges, changes to the recognition and presentation of changes in the fair value of the hedging instrument, enhancement of the ability to use the critical-terms-match method for the cash flow hedge of groups of forecasted transactions when the timing of the hedged transactions does not perfectly match the hedging instrument’s maturity date, and the addition of new disclosure requirements and amendments to existing ones. The Company applied this new guidance to hedging relationships existing on January 1, 2018, the date of adoption. The adoption of these changes did not have a material impact on the Company's financial statements, and disclosures in Note 7 reflect the requirements of this adoption.

In August 2018, the FASB issued new accounting guidance for implementation costs incurred in a cloud computing arrangement that is a service contract. This guidance specifies which implementation costs can be capitalized in a hosting arrangement that is a service contract. The guidance aligns the accounting for implementation costs for hosting arrangements, regardless of whether they convey a license to the hosted software. This guidance is required to be adopted either retrospectively or prospectively by the Company beginning in fiscal year 2020 with early adoption permitted, including adoption in any interim period. The Company early adopted this guidance in the third quarter of 2018 on a retrospective basis, which did not have a material impact on the Company’s consolidated financial statements.

Pending Accounting Pronouncements

In February 2016, the FASB issued new accounting guidance 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 ATI’s 2019 fiscal year. In July 2018, the FASB added an additional adoption method, which ATI will use to initially apply the new standard at the adoption date, January 1, 2019. This adoption method recognizes any impact to prior years’ results as a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. The Company is currently in the process of evaluating its existing lease portfolio, including accumulating all of the necessary information required to properly account for the leases under the new standard. In addition, ATI is implementing a company-wide lease management system to assist in the accounting and is evaluating additional changes to its processes and internal controls to ensure the standard’s reporting and disclosure requirements are met. The Company is currently evaluating the impact of the new guidance on its consolidated financial statements.

In August 2018, the FASB issued new disclosure guidance on fair value measurement. This new guidance modifies the disclosure requirements on fair value measurements, including removal and modifications of various current disclosures as well as some additional disclosure requirements for Level 3 fair value measurements. Some of these disclosure changes must be applied prospectively while others retrospectively depending on requirement. This guidance is required to be adopted by the Company beginning in fiscal year 2020 with early adoption permitted. The Company does not plan to early adopt this guidance. The adoption of these changes is not expected to have an impact on the Company’s consolidated financial statements other than disclosures.

In August 2018, the FASB issued new disclosure guidance for defined benefit plans, which modified certain required disclosures. This guidance is required to be adopted retrospectively by the Company beginning in fiscal year 2021, with early adoption permitted. The Company plans to early adopt this guidance in the fourth quarter of 2018, which is not expected to have an impact on the Company’s consolidated financial statements other than disclosures.