Business Combinations, Goodwill and Intangible Assets - Additional Information (Details) - USD ($)			3 Months Ended	12 Months Ended
	Nov. 30, 2020	Sep. 19, 2018	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite Lived Intangible Assets [Line Items]
Goodwill impairment			$ 25,200,000	$ 0	$ 25,200,000
Impairment associated with intangible assets				0	0	$ 0
Amortization of definite-lived intangible assets				2,900,000	$ 2,800,000	$ 1,800,000
Alcoa Warrick LLC [Member]
Finite Lived Intangible Assets [Line Items]
Business combination, purchase price	$ 670,000,000.0
Purchase price, net of cash acquired	587,000,000.0
Business combination, other post retirement benefits liabilities	$ 83,000,000.0
Acquisition costs				$ 5,500,000
Imperial Machine and Tool Co. (''IMT'') [Member]
Finite Lived Intangible Assets [Line Items]
Purchase price, net of cash acquired		$ 43,200,000