Summary of Significant Accounting Policies - Additional Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) Segment	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Significant Accounting Policies [Line Items]
Number of operating segments | Segment	1
Trade accounts receivable sold	$ 1,102.8
Net discount fees recognized	6.2
Effect of LIFO Inventory Liquidation on Income	0.2
Excess of current cost over the stated LIFO value of inventory	137.1	$ 8.4
Interest expense capitalized as construction in progress	$ 1.2	1.0	$ 1.6
Non-cash asset impairment charges		0.5	26.1
Software amortization period	9 years
Amortization expenses of cloud computing software	$ 0.1	0.0
Workers' compensation charge		0.6
Accrued employee healthcare benefits	9.0	3.1
Advertising costs	0.1	0.2	0.4
Research and development costs	$ 9.3	9.1	10.5
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
ASU No. 2021-08
Significant Accounting Policies [Line Items]
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect	true
Change in Accounting Principle, Accounting Standards Update, Early Adopted	true
Prepaid Expenses and Other Current Assets
Significant Accounting Policies [Line Items]
Hosting arrangement, service contract, implementation cost recognized	$ 1.1	0.4
Other Assets
Significant Accounting Policies [Line Items]
Hosting arrangement, service contract, implementation cost recognized	6.7	2.7
Fabricated Products | Idled Equipment
Significant Accounting Policies [Line Items]
Non-cash asset impairment charges	$ 0.0	0.5	$ 0.9
Minimum
Significant Accounting Policies [Line Items]
Number of days in maturity for debt investment securities to be classified as available for sale securities	90 days
Period over which accounts receivable is due, days	30 days
Maximum
Significant Accounting Policies [Line Items]
Period over which accounts receivable is due, days	90 days
Kaiser Aluminum Canada Limited
Significant Accounting Policies [Line Items]
Government grants income recognized from other operating charges (income), net		$ 1.3