Recently Adopted and Recently Issued Accounting Guidance	9 Months Ended
Sep. 30, 2017
Accounting Changes and Error Corrections [Abstract]
Recently Adopted and Recently Issued Accounting Guidance	B. Recently Adopted and Recently Issued Accounting Guidance Adopted On January 1, 2017, Alcoa Corporation adopted changes issued by the Financial Accounting Standards Board (FASB) to the subsequent measurement of inventory. Prior to these changes, an entity was required to measure its inventory at the lower of cost or market, whereby market can be replacement cost, net realizable value, or net realizable value less an approximately normal profit margin. The changes require that inventory be measured at the lower of cost and net realizable value, thereby eliminating the use of the other two market methodologies. Net realizable value is defined as the estimated selling prices in the ordinary course of business less reasonably predictable costs of completion, disposal, and transportation. These changes do not apply to inventories measured using LIFO or the retail inventory method. Prior to these changes, Alcoa Corporation applied the net realizable value market option to measure non-LIFO inventories at the lower of cost or market. The adoption of these changes had no impact on the Consolidated Financial Statements. On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to derivative instruments designated as hedging instruments. These changes clarify that a change in the counterparty to a derivative instrument that has been designated as a hedging instrument does not, in and of itself, require de-designation of that hedging relationship provided that all other hedge accounting criteria continue to be met. The adoption of these changes had no immediate impact on the Consolidated Financial Statements; however, this guidance will need to be considered in the event the existing counterparty to any of Alcoa Corporation’s derivative instruments changes to a new counterparty. On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to equity method investments. These changes eliminate the requirement for an investor to adjust an equity method investment, results of operations, and retained earnings retroactively on a step-by-step basis as if the equity method had been in effect during all previous periods that the investment had been held as a result of an increase in the level of ownership interest or degree of influence. Additionally, an entity that has an available-for-sale equity security that becomes qualified for the equity method of accounting must recognize through earnings the unrealized holding gain or loss in accumulated other comprehensive income at the date the investment becomes qualified for use of the equity method. The adoption of these changes had no immediate impact on the Consolidated Financial Statements; however, this guidance will need to be considered in the event any of Alcoa Corporation’s investments undergo a change as previously described. On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to employee share-based payment accounting. Prior to these changes, an entity must determine for each share-based payment award whether the difference between the deduction for tax purposes and the compensation cost recognized for financial reporting purposes results in either an excess tax benefit or a tax deficiency. Excess tax benefits are recognized in additional paid-in capital; tax deficiencies are recognized either as an offset to accumulated excess tax benefits, if any, or in the income statement. Excess tax benefits are not recognized until the deduction reduces taxes payable. The changes require all excess tax benefits and tax deficiencies related to share-based payment awards to be recognized as income tax expense or benefit in the income statement. The tax effects of exercised or vested awards should be treated as discrete items in the reporting period in which they occur. An entity also should recognize excess tax benefits regardless of whether the benefit reduces taxes payable in the current period. Additionally, the presentation of excess tax benefits related to share-based payment awards in the statement of cash flows is changed. Prior to these changes, excess tax benefits must be separated from other income tax cash flows and classified as a financing activity. The changes require excess tax benefits to be classified along with other income tax cash flows as an operating activity. Also, the changes require cash paid by an employer when directly withholding shares for tax-withholding purposes to be classified as a financing activity. Prior to these changes, there was no specific guidance on the classification in the statement of cash flows of cash paid by an employer to the tax authorities when directly withholding shares for tax-withholding purposes. Additionally, for a share-based award to qualify for equity classification it cannot partially settle in cash in excess of the employer’s minimum statutory withholding requirements. The changes permit equity classification of share-based awards for withholdings up to the maximum statutory tax rates in applicable jurisdictions. The adoption of these changes had an immaterial impact on the Consolidated Financial Statements. On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to the accounting for intra-entity transactions, other than inventory. Prior to these changes, no immediate tax impact is recognized in an entity’s financial statements as a result of intra-entity transfers of assets. An entity is precluded from reflecting a tax benefit or expense from an intra-entity asset transfer between entities that file separate tax returns, whether or not such entities are in different tax jurisdictions, until the asset has been sold to a third party or otherwise recovered. The buyer of such asset is prohibited from recognizing a deferred tax asset for the temporary difference arising from the excess of the buyer’s tax basis over the cost to the seller. The changes require the current and deferred income tax consequences of the intra-entity transfer to be recorded when the transaction occurs. The exception to defer the tax consequences of inventory transactions is maintained. The adoption of these changes had an immaterial impact on the Consolidated Financial Statements. On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to consolidation accounting. Prior to these changes, an entity was required to consider indirect economic interests in a variable interest entity held through related parties under common control as direct interests in their entirety in the entity’s assessment of whether it is the primary beneficiary of the variable interest entity. The changes result in an entity considering such indirect economic interests only on a proportionate basis as indirect interests instead of as direct interests in their entirety. The adoption of these changes had no impact on the Consolidated Financial Statements; however, this guidance will need to be considered in future assessments of whether Alcoa Corporation is the primary beneficiary of a variable interest entity. Issued In January 2017, the FASB issued changes to accounting for business combinations. These changes clarify the definition of a business for the purposes of evaluating whether a particular transaction should be accounted for as an acquisition or disposal of a business or an asset. Generally, a business is an integrated set of assets and activities that contain inputs, processes, and outputs, although outputs are not required. These changes provide a “screen” to determine whether an integrated set of assets and activities qualifies as a business. If substantially all of the fair value of the gross assets is concentrated in a single identifiable asset or a group of similar identifiable assets, the definition of a business has not been met and the transaction should be accounted for as an acquisition or disposal of an asset. Otherwise, an entity is required to evaluate whether the integrated set of assets and activities include, at a minimum, an input and a substantive process that together significantly contribute to the ability to create output and are no longer to consider whether a market participant could replace any missing elements. These changes also narrow the definition of an output. Currently, an output is defined as the ability to provide a return in the form of dividends, lower costs, or other economic benefits directly to investors, owners, members, or participants. An output would now be defined as the ability to provide goods or services to customers, investment income, or other revenues. These changes become effective for Alcoa Corporation on January 1, 2018. Management has determined that the adoption of these changes will not have an immediate impact on the Consolidated Financial Statements. This guidance will need to be considered in the event Alcoa Corporation acquires or disposes of an integrated set of assets and activities. In January 2017, the FASB issued changes to the assessment of goodwill for impairment as it relates to the quantitative test. Currently, there are two steps when performing a quantitative impairment test. The first step requires an entity to compare the current fair value of a reporting unit to its carrying value. In the event the reporting unit’s estimated fair value is less than its carrying value, an entity performs the second step, which is to compare the carrying amount of the reporting unit’s goodwill with the implied fair value of that goodwill. The implied fair value of goodwill is the excess of the fair value of the reporting unit over the fair value amounts assigned to all of the assets and liabilities of that unit as if the reporting unit was acquired in a business combination and the fair value of the reporting unit represented the purchase price. If the carrying value of goodwill exceeds its implied fair value, an impairment loss equal to such excess would be recognized. These changes eliminate the second step of the quantitative impairment test. Accordingly, an entity would recognize an impairment of goodwill for a reporting unit, if under what is currently referred to as the first step, the estimated fair value of the reporting unit is less than the carrying value. The impairment would be equal to the excess of the reporting unit’s carrying value over its fair value not to exceed the total amount of goodwill applicable to that reporting unit. These changes become effective for Alcoa Corporation on January 1, 2020. Management has determined that the adoption of these changes will not have an immediate impact on the Consolidated Financial Statements. This guidance will need to be considered each time Alcoa Corporation performs an assessment of goodwill for impairment under the quantitative test. In March 2017, the FASB issued changes to the presentation of net periodic benefit cost related to pension and other postretirement benefit plans. These changes require that an entity report the service cost component of net periodic benefit cost in the same line item(s) on the statement of operations as other compensation costs arising from services rendered by the pertinent employees during a reporting period. The other components of net periodic benefit cost (see Note J) are required to be presented separately from the service cost component. In other words, these other components may be aggregated and presented as a separate line item or they may be included in existing line items on the statement of operations other than such line items that include the service cost component. Currently, Alcoa Corporation includes all components of net periodic benefit cost in Cost of goods sold (business employees) and Selling, general administrative, and other expenses (corporate employees) consistent with the location of other compensation costs related to the respective employees. Additionally, these changes only allow the service cost component to be capitalized as applicable (e.g., as a cost of internally manufactured inventory). These changes become effective for Alcoa Corporation on January 1, 2018. Other than providing additional disclosure, management has determined that the adoption of these changes will not have a material impact on the Consolidated Financial Statements. In May 2017, the FASB issued changes to the accounting for stock-based compensation when there has been a modification to the terms or conditions of a share-based payment award. These changes require an entity to account for the modification only when there has been a substantive change to the terms or conditions of a share-based payment award. A substantive change occurs when the fair value, vesting conditions or balance sheet classification (liability or equity) of a share-based payment award is/are different immediately before and after the modification. Currently, an entity is required to account for any modification in the terms or conditions of a share-based payment award. These changes become effective for Alcoa Corporation on January 1, 2018. Management has determined that the adoption of these changes will not have an immediate impact on the Consolidated Financial Statements. This guidance will need to be considered if Alcoa Corporation initiates a modification that is determined to be a substantive change to an outstanding stock-based award. In August 2017, the FASB issued changes to the accounting for hedging activities. These changes permit hedge accounting for risk components in hedging relationships involving nonfinancial risk and interest rate risk; reduce current limitations on the designation and measurement of a hedged item in a fair value hedge of interest rate risk; remove the requirement to separately measure and report hedge ineffectiveness; provide an election to systematically and rationally recognize in earnings the initial value of any amount excluded from the assessment of hedge effectiveness for all types of hedges; and lighten the requirements of effectiveness testing. Additionally, modifications to existing disclosures, as well as additional disclosures, will be required to reflect these changes regarding the measurement and recording of hedging activities. These changes become effective for Alcoa Corporation on January 1, 2019 (early adoption is permitted). Management is currently evaluating early adopting these changes and the potential impact of these changes on the Consolidated Financial Statements. In May 2014, the FASB issued changes to the recognition of revenue from contracts with customers. These changes created a comprehensive framework for all entities in all industries to apply in the determination of when to recognize revenue, and, therefore, supersede virtually all existing revenue recognition requirements and guidance. This framework is expected to result in less complex guidance in application while providing a consistent and comparable methodology for revenue recognition. The core principle of the guidance is that 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. To achieve this principle, an entity should apply the following steps: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract(s), (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract(s), and (v) recognize revenue when, or as, the entity satisfies a performance obligation. In August 2015, the FASB deferred the effective date by one year, making these changes effective for Alcoa Corporation on January 1, 2018. Through a previously established project team, the Company has completed a detailed review of the terms and provisions of its customer contracts in light of these changes. The project team is in the process of finalizing the evaluation of these contracts under the new guidance, as well as assessing the need for any potential changes to the Company’s accounting policies and internal control structure. That said, Alcoa Corporation recognizes revenue when title, ownership, and risk of loss pass to the customer, all of which occurs upon shipment or delivery of the product and is based on the applicable shipping terms. Alcoa Corporation’s sales of its products to customers represent single performance obligations, which are not expected to be impacted by these changes. As a result, management does not expect the adoption of these changes to have a material impact on the Consolidated Financial Statements.

B. Recently Adopted and Recently Issued Accounting Guidance

Adopted

On January 1, 2017, Alcoa Corporation adopted changes issued by the Financial Accounting Standards Board (FASB) to the subsequent measurement of inventory. Prior to these changes, an entity was required to measure its inventory at the lower of cost or market, whereby market can be replacement cost, net realizable value, or net realizable value less an approximately normal profit margin. The changes require that inventory be measured at the lower of cost and net realizable value, thereby eliminating the use of the other two market methodologies. Net realizable value is defined as the estimated selling prices in the ordinary course of business less reasonably predictable costs of completion, disposal, and transportation. These changes do not apply to inventories measured using LIFO or the retail inventory method. Prior to these changes, Alcoa Corporation applied the net realizable value market option to measure non-LIFO inventories at the lower of cost or market. The adoption of these changes had no impact on the Consolidated Financial Statements.

On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to derivative instruments designated as hedging instruments. These changes clarify that a change in the counterparty to a derivative instrument that has been designated as a hedging instrument does not, in and of itself, require de-designation of that hedging relationship provided that all other hedge accounting criteria continue to be met. The adoption of these changes had no immediate impact on the Consolidated Financial Statements; however, this guidance will need to be considered in the event the existing counterparty to any of Alcoa Corporation’s derivative instruments changes to a new counterparty.

On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to equity method investments. These changes eliminate the requirement for an investor to adjust an equity method investment, results of operations, and retained earnings retroactively on a step-by-step basis as if the equity method had been in effect during all previous periods that the investment had been held as a result of an increase in the level of ownership interest or degree of influence. Additionally, an entity that has an available-for-sale equity security that becomes qualified for the equity method of accounting must recognize through earnings the unrealized holding gain or loss in accumulated other comprehensive income at the date the investment becomes qualified for use of the equity method. The adoption of these changes had no immediate impact on the Consolidated Financial Statements; however, this guidance will need to be considered in the event any of Alcoa Corporation’s investments undergo a change as previously described.

On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to employee share-based payment accounting. Prior to these changes, an entity must determine for each share-based payment award whether the difference between the deduction for tax purposes and the compensation cost recognized for financial reporting purposes results in either an excess tax benefit or a tax deficiency. Excess tax benefits are recognized in additional paid-in capital; tax deficiencies are recognized either as an offset to accumulated excess tax benefits, if any, or in the income statement. Excess tax benefits are not recognized until the deduction reduces taxes payable. The changes require all excess tax benefits and tax deficiencies related to share-based payment awards to be recognized as income tax expense or benefit in the income statement. The tax effects of exercised or vested awards should be treated as discrete items in the reporting period in which they occur. An entity also should recognize excess tax benefits regardless of whether the benefit reduces taxes payable in the current period. Additionally, the presentation of excess tax benefits related to share-based payment awards in the statement of cash flows is changed. Prior to these changes, excess tax benefits must be separated from other income tax cash flows and classified as a financing activity. The changes require excess tax benefits to be classified along with other income tax cash flows as an operating activity. Also, the changes require cash paid by an employer when directly withholding shares for tax-withholding purposes to be classified as a financing activity. Prior to these changes, there was no specific guidance on the classification in the statement of cash flows of cash paid by an employer to the tax authorities when directly withholding shares for tax-withholding purposes. Additionally, for a share-based award to qualify for equity classification it cannot partially settle in cash in excess of the employer’s minimum statutory withholding requirements. The changes permit equity classification of share-based awards for withholdings up to the maximum statutory tax rates in applicable jurisdictions. The adoption of these changes had an immaterial impact on the Consolidated Financial Statements.

On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to the accounting for intra-entity transactions, other than inventory. Prior to these changes, no immediate tax impact is recognized in an entity’s financial statements as a result of intra-entity transfers of assets. An entity is precluded from reflecting a tax benefit or expense from an intra-entity asset transfer between entities that file separate tax returns, whether or not such entities are in different tax jurisdictions, until the asset has been sold to a third party or otherwise recovered. The buyer of such asset is prohibited from recognizing a deferred tax asset for the temporary difference arising from the excess of the buyer’s tax basis over the cost to the seller. The changes require the current and deferred income tax consequences of the intra-entity transfer to be recorded when the transaction occurs. The exception to defer the tax consequences of inventory transactions is maintained. The adoption of these changes had an immaterial impact on the Consolidated Financial Statements.

On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to consolidation accounting. Prior to these changes, an entity was required to consider indirect economic interests in a variable interest entity held through related parties under common control as direct interests in their entirety in the entity’s assessment of whether it is the primary beneficiary of the variable interest entity. The changes result in an entity considering such indirect economic interests only on a proportionate basis as indirect interests instead of as direct interests in their entirety. The adoption of these changes had no impact on the Consolidated Financial Statements; however, this guidance will need to be considered in future assessments of whether Alcoa Corporation is the primary beneficiary of a variable interest entity.

Issued

In January 2017, the FASB issued changes to accounting for business combinations. These changes clarify the definition of a business for the purposes of evaluating whether a particular transaction should be accounted for as an acquisition or disposal of a business or an asset. Generally, a business is an integrated set of assets and activities that contain inputs, processes, and outputs, although outputs are not required. These changes provide a “screen” to determine whether an integrated set of assets and activities qualifies as a business. If substantially all of the fair value of the gross assets is concentrated in a single identifiable asset or a group of similar identifiable assets, the definition of a business has not been met and the transaction should be accounted for as an acquisition or disposal of an asset. Otherwise, an entity is required to evaluate whether the integrated set of assets and activities include, at a minimum, an input and a substantive process that together significantly contribute to the ability to create output and are no longer to consider whether a market participant could replace any missing elements. These changes also narrow the definition of an output. Currently, an output is defined as the ability to provide a return in the form of dividends, lower costs, or other economic benefits directly to investors, owners, members, or participants. An output would now be defined as the ability to provide goods or services to customers, investment income, or other revenues. These changes become effective for Alcoa Corporation on January 1, 2018. Management has determined that the adoption of these changes will not have an immediate impact on the Consolidated Financial Statements. This guidance will need to be considered in the event Alcoa Corporation acquires or disposes of an integrated set of assets and activities.

In January 2017, the FASB issued changes to the assessment of goodwill for impairment as it relates to the quantitative test. Currently, there are two steps when performing a quantitative impairment test. The first step requires an entity to compare the current fair value of a reporting unit to its carrying value. In the event the reporting unit’s estimated fair value is less than its carrying value, an entity performs the second step, which is to compare the carrying amount of the reporting unit’s goodwill with the implied fair value of that goodwill. The implied fair value of goodwill is the excess of the fair value of the reporting unit over the fair value amounts assigned to all of the assets and liabilities of that unit as if the reporting unit was acquired in a business combination and the fair value of the reporting unit represented the purchase price. If the carrying value of goodwill exceeds its implied fair value, an impairment loss equal to such excess would be recognized. These changes eliminate the second step of the quantitative impairment test. Accordingly, an entity would recognize an impairment of goodwill for a reporting unit, if under what is currently referred to as the first step, the estimated fair value of the reporting unit is less than the carrying value. The impairment would be equal to the excess of the reporting unit’s carrying value over its fair value not to exceed the total amount of goodwill applicable to that reporting unit. These changes become effective for Alcoa Corporation on January 1, 2020. Management has determined that the adoption of these changes will not have an immediate impact on the Consolidated Financial Statements. This guidance will need to be considered each time Alcoa Corporation performs an assessment of goodwill for impairment under the quantitative test.

In March 2017, the FASB issued changes to the presentation of net periodic benefit cost related to pension and other postretirement benefit plans. These changes require that an entity report the service cost component of net periodic benefit cost in the same line item(s) on the statement of operations as other compensation costs arising from services rendered by the pertinent employees during a reporting period. The other components of net periodic benefit cost (see Note J) are required to be presented separately from the service cost component. In other words, these other components may be aggregated and presented as a separate line item or they may be included in existing line items on the statement of operations other than such line items that include the service cost component. Currently, Alcoa Corporation includes all components of net periodic benefit cost in Cost of goods sold (business employees) and Selling, general administrative, and other expenses (corporate employees) consistent with the location of other compensation costs related to the respective employees. Additionally, these changes only allow the service cost component to be capitalized as applicable (e.g., as a cost of internally manufactured inventory). These changes become effective for Alcoa Corporation on January 1, 2018. Other than providing additional disclosure, management has determined that the adoption of these changes will not have a material impact on the Consolidated Financial Statements.

In May 2017, the FASB issued changes to the accounting for stock-based compensation when there has been a modification to the terms or conditions of a share-based payment award. These changes require an entity to account for the modification only when there has been a substantive change to the terms or conditions of a share-based payment award. A substantive change occurs when the fair value, vesting conditions or balance sheet classification (liability or equity) of a share-based payment award is/are different immediately before and after the modification. Currently, an entity is required to account for any modification in the terms or conditions of a share-based payment award. These changes become effective for Alcoa Corporation on January 1, 2018. Management has determined that the adoption of these changes will not have an immediate impact on the Consolidated Financial Statements. This guidance will need to be considered if Alcoa Corporation initiates a modification that is determined to be a substantive change to an outstanding stock-based award.

In August 2017, the FASB issued changes to the accounting for hedging activities. These changes permit hedge accounting for risk components in hedging relationships involving nonfinancial risk and interest rate risk; reduce current limitations on the designation and measurement of a hedged item in a fair value hedge of interest rate risk; remove the requirement to separately measure and report hedge ineffectiveness; provide an election to systematically and rationally recognize in earnings the initial value of any amount excluded from the assessment of hedge effectiveness for all types of hedges; and lighten the requirements of effectiveness testing. Additionally, modifications to existing disclosures, as well as additional disclosures, will be required to reflect these changes regarding the measurement and recording of hedging activities. These changes become effective for Alcoa Corporation on January 1, 2019 (early adoption is permitted). Management is currently evaluating early adopting these changes and the potential impact of these changes on the Consolidated Financial Statements.

In May 2014, the FASB issued changes to the recognition of revenue from contracts with customers. These changes created a comprehensive framework for all entities in all industries to apply in the determination of when to recognize revenue, and, therefore, supersede virtually all existing revenue recognition requirements and guidance. This framework is expected to result in less complex guidance in application while providing a consistent and comparable methodology for revenue recognition. The core principle of the guidance is that 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. To achieve this principle, an entity should apply the following steps: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract(s), (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract(s), and (v) recognize revenue when, or as, the entity satisfies a performance obligation. In August 2015, the FASB deferred the effective date by one year, making these changes effective for Alcoa Corporation on January 1, 2018. Through a previously established project team, the Company has completed a detailed review of the terms and provisions of its customer contracts in light of these changes. The project team is in the process of finalizing the evaluation of these contracts under the new guidance, as well as assessing the need for any potential changes to the Company’s accounting policies and internal control structure. That said, Alcoa Corporation recognizes revenue when title, ownership, and risk of loss pass to the customer, all of which occurs upon shipment or delivery of the product and is based on the applicable shipping terms. Alcoa Corporation’s sales of its products to customers represent single performance obligations, which are not expected to be impacted by these changes. As a result, management does not expect the adoption of these changes to have a material impact on the Consolidated Financial Statements.