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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 |

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