v3.20.2

Contingencies and Commitments​ (Details)			1 Months Ended	3 Months Ended	9 Months Ended
	Jun. 06, 2019 plaintiff	Sep. 15, 2017 executive	Jun. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) firefighters policeOfficers	Sep. 30, 2020 USD ($) firefighters policeOfficers	Jun. 26, 2020 claimants	Jun. 12, 2020 firefighters policeOfficers claimants	Jun. 11, 2020 firefighters	Dec. 31, 2019 USD ($)
Site Contingency [Line Items]
Environmental remediation				$ 181,000,000	$ 181,000,000				$ 208,000,000
Environmental remediation, current				97,000,000	97,000,000				83,000,000
Environmental remediation, payments				33,000,000	54,000,000
Accrual for Environmental Loss Contingencies, Increase (Decrease) for Acquisitions and Divestitures					13,000,000
Accrual for Environmental Loss Contingencies, Period Increase (Decrease)				$ 1,000,000	$ 3,000,000
Loss Contingency, Number of Firefighters | firefighters				1	1		2	80
Accrual for Environmental Loss Contingencies, Revision in Estimates					$ 5,000,000
Loss Contingency, Number of Police Officers | policeOfficers				37	37		27
Site Remediation Systems [Member]
Site Contingency [Line Items]
Environmental remediation, additions				$ (1,000,000)
Unpaid Invoices [Member]
Site Contingency [Line Items]
Accounts Payable, Trade									22,000,000
Other Environmental [Member]
Site Contingency [Line Items]
Accrual for Environmental Loss Contingencies, Period Increase (Decrease)					$ (5,000,000)
Aluminum Rolling Mill in Itapissuma, Brazil
Site Contingency [Line Items]
Environmental remediation, additions					(1,000,000)
Behrens v. Arconic
Site Contingency [Line Items]
Number of plaintiffs | plaintiff	247
Sullivan v. Arconic
Site Contingency [Line Items]
Number of former executives | executive		3
Birnberg Peirce Ltd [Member]
Site Contingency [Line Items]
Loss Contingency, Number of Claimants | claimants							4
Howe & Co Solicitors [Member]
Site Contingency [Line Items]
Loss Contingency, Number of Claimants | claimants							2
Russell-Cooke LLP [Member]
Site Contingency [Line Items]
Loss Contingency, Number of Claimants | claimants						3
Massena West, NY
Site Contingency [Line Items]
Environmental remediation				$ 131,000,000	$ 131,000,000				171,000,000
Environmental remediation, additions			$ (25,000,000)
Accounts Payable, Trade									$ 21,000,000

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