Recently Adopted and Recently Issued Accounting Guidance (Policies)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Recent Accounting Guidance	Adopted On January 1, 2021, Arconic adopted changes issued by the Financial Accounting Standards Board (FASB) to accounting for income taxes. The FASB issued this guidance to simplify various aspects related to the accounting for income taxes as part of the overall initiative to reduce complexity in accounting standards. These changes include the removal of certain exceptions and the simplification of several provisions, including: requiring an entity to recognize tax that is partially based on income, such as franchise tax, as income-based tax and account for additional amounts incurred as non-income based tax; requiring an entity to evaluate when a step up in tax basis of goodwill should be considered part of the original business combination or a separate transaction; and requiring an entity to reflect the effect of an enacted change in tax laws or rates in the annual effective tax rate computation in the interim period that includes the enactment date. The adoption of this guidance did not have a material impact on the Consolidated Financial Statements. The Company will need to continue to consider this guidance in future periods. Issued In March 2020, the FASB issued guidance that provides optional expedients and exceptions for a limited period of time to ease the potential burden in accounting for (or recognizing the effects of) reference rate reform on financial reporting. These expedients and exceptions may be used when applying GAAP, if certain criteria are met, to contracts, hedging relationships, and other transactions that reference LIBOR or another reference rate expected to be discontinued because of such reform. The purpose of this guidance is to provide relief to entities from experiencing unintended accounting and/or financial reporting outcomes or consequences due to reference rate reform. This guidance became effective immediately on March 12, 2020 and may be applied prospectively to contract modifications made and hedging relationships entered into or evaluated on or before December 31, 2022, after which time the expedients and exceptions expire. In January 2021, the FASB issued clarifying guidance to specify that certain of the optional expedients and exceptions apply to derivatives that use an interest rate for margining, discounting, or contract price alignment that is modified as a result of reference rate reform. This additional guidance may be applied on a full retrospective basis as of any date from the beginning of an interim period that includes or is subsequent to March 12, 2020, or prospectively in the manner previously described for the guidance issued on March 12, 2020. As of September 30, 2021, Arconic has not experienced any unintended outcomes or consequences of reference rate reform that would necessitate the adoption of this guidance. Additionally, the Company will not need to consider the application of this guidance related to its credit agreement, which is scheduled to mature on May 13, 2025 and provides a credit facility that is referenced to LIBOR in certain borrowing situations, as the terms of such agreement currently provide for a replacement rate if LIBOR is discontinued by the end of 2021 as expected. That said, management will continue to closely monitor all potential instances of reference rate reform to determine if adoption of this guidance becomes necessary in the future.
Consolidation, Policy	Principles of Consolidation. The Company’s Consolidated Financial Statements include the accounts of Arconic and companies in which Arconic has a controlling interest. Intercompany transactions have been eliminated. Management evaluates whether an Arconic entity or interest is a variable interest entity and whether the Company is the primary beneficiary. Consolidation is required if both of these criteria are met. Arconic does not have any variable interest entities requiring consolidation. Prior to the Separation Date, Arconic did not operate as a separate, standalone entity. Arconic’s operations were included in ParentCo’s financial results. Accordingly, for all periods prior to the Separation Date, the accompanying Consolidated Financial Statements were prepared from ParentCo’s historical accounting records and were presented on a standalone basis as if the Arconic Corporation Businesses had been conducted independently from ParentCo. Such Consolidated Financial Statements include the historical operations that were considered to comprise the Arconic Corporation Businesses, as well as certain assets and liabilities that were historically held at ParentCo’s corporate level but were specifically identifiable or otherwise attributable to Arconic. ParentCo’s net investment in these operations was reflected as Parent Company net investment on the Company’s Consolidated Financial Statements. All significant transactions and accounts within Arconic were eliminated. All significant intercompany transactions between ParentCo and Arconic were included within Parent Company net investment on Arconic’s Consolidated Financial Statements.
Costs Associated with Exit or Disposal Activities or Restructurings, Policy	Cost Allocations. The description and information on cost allocations is applicable for all periods included in the Company’s Consolidated Financial Statements prior to the Separation Date. Arconic’s Consolidated Financial Statements include general corporate expenses of ParentCo that were not historically charged to the Arconic Corporation Businesses for certain support functions that were provided on a centralized basis, such as expenses related to finance, audit, legal, information technology, human resources, communications, compliance, facilities, employee benefits and compensation, and research and development activities. These general corporate expenses were included on the accompanying Statement of Consolidated Operations within Cost of goods sold, Selling, general administrative and other expenses, and Research and development expenses. These expenses were allocated to Arconic on the basis of direct usage when identifiable, with the remainder allocated based on the Arconic Corporation Businesses’ segment revenue as a percentage of ParentCo’s total segment revenue, as reported in the respective periods. All external debt not directly attributable to the Arconic Corporation Businesses was excluded from the Company’s Consolidated Balance Sheet. Financing costs related to these debt obligations were allocated to Arconic based on the ratio of capital invested by ParentCo in the Arconic Corporation Businesses to the total capital invested by ParentCo in both the Arconic Corporation Businesses and the Howmet Aerospace Businesses, and were included on the accompanying Statement of Consolidated Operations within Interest expense.