Actual remediation fieldwork period.

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Amount Committed Under Outstanding Surety Bonds

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Income tax examination assessment outstanding.

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Number of cleanup locations.

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Percentage of cost of goods sold.

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Tax Agreement, Indemnification of Ultimate Liability, Percent

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Total costs accrued as of the balance sheet date for environmental loss contingencies.

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Amount of cash outflow to settle environmental loss contingencies.

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Carrying value of the obligation (known or estimated) arising from requirements to perform activities to remediate one or more sites, payable in twelve months or in the next operating cycle if longer.

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The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.

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Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

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Fair value portion of guarantee, including, but not limited to, financial standby letter of credit, fair value guarantee, guarantee of collection of scheduled contractual cash flows from financial assets, and contracts that contingently require the guarantor to make payments or perform services.

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Carrying amount as of the balance sheet date of income taxes previously overpaid to tax authorities (such as U.S. Federal, state and local tax authorities) representing refunds of overpayments or recoveries based on agreed-upon resolutions of disputes. Also called income tax refund receivable.

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of increase in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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Contingencies and Commitments (Details) € in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 EUR (€)	Sep. 30, 2018 USD ($) location	Sep. 30, 2018 EUR (€)	Jul. 31, 2018 USD ($)	Jul. 31, 2018 EUR (€)	Dec. 31, 2017 USD ($)	Nov. 01, 2016 USD ($)
Loss Contingencies [Line Items]
Number of cleanup locations \| location			100
Remediation reserve balance	$ 280		$ 280				$ 294
Remediation reserve balance, classified as a current liability	70		70				41
Payments related to remediation expenses applied against the reserve	7		$ 15
Actual remediation fieldwork period			4 years
Income tax examination assessment outstanding					$ 175	€ 152
Guarantees of third party related to project financing	32		$ 32
Long-term debt	6,318		6,318				6,807
Total amount committed under outstanding surety bonds	49		49
Bank Loan Obligations \| Letter of Credit
Loss Contingencies [Line Items]
Long-term debt	129		129
Separation Agreement
Loss Contingencies [Line Items]
Total amount committed under outstanding surety bonds	25		25
Other Noncurrent Liabilities and Deferred Credits
Loss Contingencies [Line Items]
Combined fair value of guarantees	7		7				8
Massena West, NY
Loss Contingencies [Line Items]
Remediation reserve balance	208		208				215
Spain
Loss Contingencies [Line Items]
Established tax reserve	59	€ 51
Tax indemnification receivable	$ 29		$ 29	€ 25
Alcoa Corporation
Loss Contingencies [Line Items]
Tax agreement, indemnification of ultimate liability, percent	49.00%		49.00%	49.00%
Recurring Costs of Managing Hazardous Substances and Environmental Programs \| Maximum
Loss Contingencies [Line Items]
Percentage of cost of goods sold			1.00%
Supply Agreement
Loss Contingencies [Line Items]
Guarantees of third party related to project financing	$ 1,141		$ 1,141				$ 1,297
Alcoa Corporation Workers Compensation Claims
Loss Contingencies [Line Items]
Letters of credit, total amount committed								$ 53