Business Combinations, Goodwill and Intangible Assets - Schedule of Preliminary Purchase Consideration (Details) - USD ($) $ in Millions			6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2021
Business Acquisition [Line Items]
Cash paid at acquisition close on March 31, 2021			$ 617.5
Alcoa Warrick LLC [Member]
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
Estimate of post-close adjustments	(8.0)
Total preliminary purchase consideration to allocate	$ 609.5