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Subsequent Events	9 Months Ended
Subsequent Events	Sep. 30, 2018
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events Management evaluated all activity of Arconic and concluded that no subsequent events have occurred that would require recognition in the Consolidated Financial Statements or disclosure in the Notes to the Consolidated Financial Statements, except as noted below: See Note G for details of a subsequent event related to certain U.S. pension plan lump sum payments and see Note P for details of a subsequent event related to the divestiture of the Texarkana, Texas rolling mill.