Business Combinations, Goodwill and Intangible Assets - Summary of Preliminary Fair Value of Assets Acquired and Liabilities Assumed as of Acquisition Date (Details) - USD ($) $ in Millions	Sep. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Finite Lived Intangible Assets [Line Items]
Intangible assets	$ 50.0
Goodwill	39.3		$ 18.8
Alcoa Warrick LLC [Member]
Finite Lived Intangible Assets [Line Items]
Trade receivables	160.0	$ 162.4
Other receivables	11.0	6.2
Inventories, including step up	209.1	209.0
Prepaid expenses and other current assets	0.3
Property, plant and equipment	343.7	385.4
Operating lease assets	12.3	12.3
Intangible assets	50.0	56.5
Goodwill	20.5	27.4
Other Assets	56.8
Accounts payable	(143.2)	(143.0)
Accrued salaries, wages and related expenses	(6.9)	(5.9)
Other accrued liabilities	(10.6)	(11.5)
Long-term portion of operating lease liabilities	(8.7)	(8.7)
Pension and other postretirement benefits	(83.5)	(83.7)
Long-term liabilities	(1.6)	(1.6)
Preliminary allocated purchase consideration	609.2	$ 604.8
Alcoa Warrick LLC [Member] | Adjustments
Finite Lived Intangible Assets [Line Items]
Trade receivables	(2.4)
Other receivables	4.8
Inventories, including step up	0.1
Prepaid expenses and other current assets	0.3
Property, plant and equipment	(41.7)
Intangible assets	(6.5)
Goodwill	(6.9)
Other Assets	56.8
Accounts payable	(0.2)
Accrued salaries, wages and related expenses	(1.0)
Other accrued liabilities	0.9
Pension and other postretirement benefits	0.2
Preliminary allocated purchase consideration	$ 4.4