Recently Adopted and Recently Issued Accounting Guidance	9 Months Ended
Sep. 30, 2019
New Accounting Pronouncements And Changes In Accounting Principles [Abstract]
Recently Adopted and Recently Issued Accounting Guidance	B. Recently Adopted and Recently Issued Accounting Guidance Adopted On January 1, 2019 Alcoa Corporation adopted Accounting Standards Update (ASU) No. 2016-02, Leases, issued by the Financial Accounting Standards Board (FASB) regarding the accounting for leases, using the modified retrospective approach. This ASU requires lessees to recognize a right-of-use asset and lease liability on the balance sheet for operating and finance leases with a term of 12 months or more. Additionally, when measuring assets and liabilities arising from a lease, optional payments should be included only if the lessee is reasonably certain to exercise an option to extend the lease, exercise a purchase option, or not exercise an option to terminate the lease. A right-of-use asset represents an entity’s right to use the underlying asset for the lease term, and a lease liability represents an entity’s obligation to make lease payments. The Company has made a policy election not to record any non-lease components in the lease liability. Previously, an asset and liability were only recorded for leases classified as capital leases (financing leases). The measurement, recognition, and presentation of expenses and cash flows arising from leases by a lessee remains the same. Management elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allowed carry forward of historical lease classifications. Additionally, in July 2018, the FASB issued ASU No. 2018-11, Targeted Improvements, to provide for an alternative transition method to the new lease guidance, whereby an entity can choose to not reflect the impact of the new lease guidance in the prior periods included in its financial statements. The Company elected this alternative transition method upon adoption on January 1, 2019. Management also elected the practical expedient related to land easements, allowing the Company to carry forward the current treatment on existing arrangements. As a result of the adoption, management recorded a right-of-use asset and lease liability, each in the amount of $201, on Alcoa Corporation’s Consolidated Balance Sheet as of January 1, 2019 for several types of operating leases, including land and buildings, alumina refinery process control technology, plant equipment, vehicles, and computer equipment. See Note L for additional information related to the adoption of this standard. Alcoa Corporation’s adoption of the following accounting guidance in 2019 did not have a material impact on the Company’s Consolidated Financial Statements: Accounting Standards Update 2018-01 Leases: Land Easement Practical Expedient for Transition 2018-02 Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income 2018-07 Stock Compensation: Improvements to Nonemployee Share-Based Payment Accounting Issued In August 2018, the FASB issued ASU No. 2018-14, Compensation - Retirement Benefits - Defined Benefit Plans - General. This ASU makes changes to the disclosures of fair value measurements and defined benefit plans through several removals, modifications, additions, and/or clarifications of the existing requirements. Certain disclosures associated with accumulated other comprehensive income, valuation of Level 3 assets, and sensitivities in assumed health care trend rates and interest rates have been eliminated. New disclosures have been added to explain significant gains and losses related to changes in benefit obligations, changes included in other comprehensive income for recurring Level 3 fair value measurements, and information on significant unobservable inputs used to develop Level 3 fair value measurements. These changes become effective for Alcoa Corporation for its fiscal year ending December 31, 2020 and for interim periods therein with early adoption permitted and retrospective presentation for all periods presented required. Other than updating the applicable disclosures, the adoption of this guidance will not have an impact on the Company’s Consolidated Financial Statements. In August 2018, the FASB issued ASU No. 2018-15, Intangibles - Goodwill and Other - Internal-Use Software. This ASU aligns the accounting for cloud computing implementation costs with that of costs to develop or obtain internal-use software, meaning such costs that are part of the application development stage are capitalized as an asset and amortized over the term of the arrangement, otherwise, such costs are expensed as incurred. It also clarifies the classification of amounts related to capitalized implementation costs in the financial statements. This guidance becomes effective for Alcoa Corporation on January 1, 2020, with early adoption permitted. Management has completed our assessment of the impact related to this guidance and concluded that the adoption of this guidance will not have a material impact on the Company’s Consolidated Financial Statements. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses. This ASU added a new impairment model (known as the current expected credit loss (CECL) model) that is based on expected losses rather than incurred losses. Under the new guidance, an entity recognizes an allowance for its estimate of expected credit losses and applies to most debt instruments, trade receivables, lease receivables, financial guarantee contracts, and other loan commitments. The CECL model does not have a minimum threshold for recognition of impairment losses and entities will need to measure expected credit losses on assets that have a low risk of loss. These changes become effective for Alcoa Corporation on January 1, 2020. Management is finalizing our assessment of the impact of these changes on the Company’s Consolidated Financial Statements but does not expect a material impact from the adoption of this guidance, as our current loss models incorporate both historic and forward-looking information.

Recently Adopted and Recently Issued Accounting Guidance

9 Months Ended

Sep. 30, 2019

New Accounting Pronouncements And Changes In Accounting Principles [Abstract]

Recently Adopted and Recently Issued Accounting Guidance

B. Recently Adopted and Recently Issued Accounting Guidance

Adopted

On January 1, 2019 Alcoa Corporation adopted Accounting Standards Update (ASU) No. 2016-02, Leases, issued by the Financial Accounting Standards Board (FASB) regarding the accounting for leases, using the modified retrospective approach. This ASU requires lessees to recognize a right-of-use asset and lease liability on the balance sheet for operating and finance leases with a term of 12 months or more. Additionally, when measuring assets and liabilities arising from a lease, optional payments should be included only if the lessee is reasonably certain to exercise an option to extend the lease, exercise a purchase option, or not exercise an option to terminate the lease. A right-of-use asset represents an entity’s right to use the underlying asset for the lease term, and a lease liability represents an entity’s obligation to make lease payments. The Company has made a policy election not to record any non-lease components in the lease liability. Previously, an asset and liability were only recorded for leases classified as capital leases (financing leases). The measurement, recognition, and presentation of expenses and cash flows arising from leases by a lessee remains the same. Management elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allowed carry forward of historical lease classifications. Additionally, in July 2018, the FASB issued ASU No. 2018-11, Targeted Improvements, to provide for an alternative transition method to the new lease guidance, whereby an entity can choose to not reflect the impact of the new lease guidance in the prior periods included in its financial statements. The Company elected this alternative transition method upon adoption on January 1, 2019. Management also elected the practical expedient related to land easements, allowing the Company to carry forward the current treatment on existing arrangements.

As a result of the adoption, management recorded a right-of-use asset and lease liability, each in the amount of $201, on Alcoa Corporation’s Consolidated Balance Sheet as of January 1, 2019 for several types of operating leases, including land and buildings, alumina refinery process control technology, plant equipment, vehicles, and computer equipment. See Note L for additional information related to the adoption of this standard.

Alcoa Corporation’s adoption of the following accounting guidance in 2019 did not have a material impact on the Company’s Consolidated Financial Statements:

Accounting Standards Update

2018-01 Leases: Land Easement Practical Expedient for Transition

2018-02 Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income

2018-07 Stock Compensation: Improvements to Nonemployee Share-Based Payment Accounting

Issued

In August 2018, the FASB issued ASU No. 2018-14, Compensation - Retirement Benefits - Defined Benefit Plans - General. This ASU makes changes to the disclosures of fair value measurements and defined benefit plans through several removals, modifications, additions, and/or clarifications of the existing requirements. Certain disclosures associated with accumulated other comprehensive income, valuation of Level 3 assets, and sensitivities in assumed health care trend rates and interest rates have been eliminated. New disclosures have been added to explain significant gains and losses related to changes in benefit obligations, changes included in other comprehensive income for recurring Level 3 fair value measurements, and information on significant unobservable inputs used to develop Level 3 fair value measurements. These changes become effective for Alcoa Corporation for its fiscal year ending December 31, 2020 and for interim periods therein with early adoption permitted and retrospective presentation for all periods presented required. Other than updating the applicable disclosures, the adoption of this guidance will not have an impact on the Company’s Consolidated Financial Statements.

In August 2018, the FASB issued ASU No. 2018-15, Intangibles - Goodwill and Other - Internal-Use Software. This ASU aligns the accounting for cloud computing implementation costs with that of costs to develop or obtain internal-use software, meaning such costs that are part of the application development stage are capitalized as an asset and amortized over the term of the arrangement, otherwise, such costs are expensed as incurred. It also clarifies the classification of amounts related to capitalized implementation costs in the financial statements. This guidance becomes effective for Alcoa Corporation on January 1, 2020, with early adoption permitted. Management has completed our assessment of the impact related to this guidance and concluded that the adoption of this guidance will not have a material impact on the Company’s Consolidated Financial Statements.

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses. This ASU added a new impairment model (known as the current expected credit loss (CECL) model) that is based on expected losses rather than incurred losses. Under the new guidance, an entity recognizes an allowance for its estimate of expected credit losses and applies to most debt instruments, trade receivables, lease receivables, financial guarantee contracts, and other loan commitments. The CECL model does not have a minimum threshold for recognition of impairment losses and entities will need to measure expected credit losses on assets that have a low risk of loss. These changes become effective for Alcoa Corporation on January 1, 2020. Management is finalizing our assessment of the impact of these changes on the Company’s Consolidated Financial Statements but does not expect a material impact from the adoption of this guidance, as our current loss models incorporate both historic and forward-looking information.