Business Combinations, Goodwill and Intangible Assets - Schedule of Preliminary Purchase Consideration (Details) - Alcoa Warrick LLC [Member] - USD ($) $ in Millions	Sep. 30, 2021	Mar. 31, 2021
Business Acquisition [Line Items]
Contract purchase price	$ 670.0	$ 670.0
Preliminary working capital adjustment	31.0
Initial outstanding indebtedness – Other postretirement benefits liabilities	(83.5)
Cash paid at acquisition close on March 31, 2021	617.5
Cash received for post-close adjustments	(8.3)
Total purchase consideration to allocate	$ 609.2