Business Combinations, Goodwill and Intangible Assets - Schedule of Goodwill (Details) - USD ($) $ in Millions	Sep. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Finite Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 57.7		$ 37.2
Accumulated Impairment Loss	(18.4)		(18.4)
Net Carrying Value	39.3		$ 18.8
Alcoa Warrick LLC [Member]
Finite Lived Intangible Assets [Line Items]
Gross Carrying Value	20.5
Net Carrying Value	$ 20.5	$ 27.4

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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