Basis of Presentation and Recent Accounting Pronouncements - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Mar. 31, 2021
Significant Accounting Policies [Line Items]
Trade accounts receivable sold	$ 390.2		$ 706.2
Net discount fees recognized	$ 2.1		$ 4.0
ASU No. 2019-12
Significant Accounting Policies [Line Items]
Change in Accounting Principle, Accounting Standards Update, Adopted					true
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect					true
Kaiser Aluminum Canada Limited
Significant Accounting Policies [Line Items]
Government grants income recognized from other operating charges (income), net		$ 0.1		$ 1.3