Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Name:	us-gaap_GoodwillImpairmentLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Basis of Presentation (Details)	3 Months Ended	6 Months Ended	12 Months Ended
Basis of Presentation (Details)	Mar. 31, 2018 USD ($)	Jun. 30, 2018 USD ($) reporting_unit	Dec. 31, 2017 USD ($) reporting_unit
Basis Of Presentation [Line Items]
Number of reporting units \| reporting_unit		3	4
Goodwill, impairment loss		$ 0
Goodwill		$ 4,518,000,000	$ 4,535,000,000
Arconic Engines
Basis Of Presentation [Line Items]
Goodwill	$ 2,095,000,000
Alcoa Fastening Systems And Rings
Basis Of Presentation [Line Items]
Goodwill	1,623,000,000
Arconic Engineered Structures
Basis Of Presentation [Line Items]
Goodwill	$ 517,000,000
Minimum
Basis Of Presentation [Line Items]
Percentage of goodwill allocated to reporting units	92.00%