Business Combinations, Goodwill and Intangible Assets - Summary of Preliminary Fair Value of Assets Acquired and Liabilities Assumed at Acquisition Date (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Finite Lived Intangible Assets [Line Items]
Intangible assets	$ 56.5
Goodwill	46.2	$ 18.8
Alcoa Warrick LLC [Member]
Finite Lived Intangible Assets [Line Items]
Trade receivables	162.4
Other receivables	6.2
Inventories, including step up	209.0
Property, plant and equipment	385.4
Operating lease assets	12.3
Intangible assets	56.5
Goodwill	27.4
Accounts payable	(143.0)
Accrued salaries, wages and related expenses	(5.9)
Other accrued liabilities	(11.5)
Long-term portion of operating lease liabilities	(8.7)
Pension and other postretirement benefits	83.7
Long-term liabilities	(1.6)
Preliminary allocated purchase consideration	$ 604.8