Business Combinations, Goodwill and Intangible Assets - Summary of Finalized Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Finite Lived Intangible Assets [Line Items]
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	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)
Total 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	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
Total allocated purchase consideration	$ 4.4