Debt and Credit Facility	12 Months Ended
Debt and Credit Facility	Dec. 31, 2019
Debt Disclosure [Abstract]
Debt and Credit Facility	Debt and Credit Facility Senior Notes 4.625% Senior Notes. In November 2019, we issued $500.0 million aggregate principal amount of our 4.625% unsecured senior notes due March 2028 ("4.625% Senior Notes") at 100% of the principal amount. The unamortized amount of debt issuance costs relating to the 4.625% Senior Notes as of December 31, 2019 was $7.5 million. Interest expense, including amortization of debt issuance costs, relating to the 4.625% Senior Notes was $2.4 million for the year ended December 31, 2019. Interest accrues on the 4.625% Senior Notes beginning November 26, 2019 at a rate of 4.625% per annum and is payable semiannually on March 1 and September 1 of each year. A portion of the interest relating to the 4.625% Senior Notes was capitalized as construction in progress. The effective interest rate of the 4.625% Senior Notes is approximately 4.8% per annum, taking into account the amortization of debt issuance costs. The 4.625% Senior Notes were offered and sold in transactions not required to be registered under the Securities Act of 1933 and are not entitled to any registration rights. The fair value of the outstanding 4.625% Senior Notes, which are Level 1 liabilities calculated based on pricing from trades around the balance sheet date, was approximately $513.5 million at December 31, 2019. The 4.625% Senior Notes are unsecured obligations and are guaranteed by each of our existing and future domestic subsidiaries that is a borrower or guarantor under the revolving credit facility (see Revolving Credit Facility below). The indenture governing the 4.625% Senior Notes ("indenture") places limitations on our and our restricted subsidiaries' ability to, among other things: (i) incur liens; (ii) consolidate, merge or sell all or substantially all of our and our restricted subsidiaries' assets; (iii) incur or guarantee additional indebtedness; (iv) enter into transactions with affiliates; and (v) make "restricted payments" (as defined in the indenture to include certain loans, investments, dividend payments, share repurchases and prepayments, redemptions and repurchases of certain indebtedness). Certain types and amounts of restricted payments are allowed by various provisions of the indenture. In particular, the indenture provisions permit us to make restricted payments in any amount if, after giving effect to such restricted payments, our "consolidated net debt ratio" (as defined in the indenture) equals or is less than 2.75:1.00. We may redeem the 4.625% Senior Notes at our option in whole or part at any time on or after March 1, 2023 at redemption prices (expressed as percentages of principal amount) of 102.313%, declining to 101.156% and 100% on or after March 1, 2024 and March 1, 2025, respectively, in each case plus any accrued and unpaid interest to the applicable redemption date. At any time prior to March 1, 2023, we may also redeem some or all of the 4.625% Senior Notes at a redemption price equal to 100% of the principal amount of the 4.625% Senior Notes redeemed, plus the "applicable premium" (as defined in the indenture) as of the redemption date and any accrued and unpaid interest thereon. In addition, subject to the conditions set forth in the indenture, at any time prior to March 1, 2023, we may also redeem up to 40.0% of the 4.625% Senior Notes using the net proceeds from certain equity offerings at a redemption price equal to 104.625% of the principal amount plus any accrued and unpaid interest. Holders of the 4.625% Senior Notes have the right to require us to repurchase the 4.625% Senior Notes at a price in cash equal to 101% of the aggregate principal amount plus any accrued and unpaid interest following the occurrence of both: (i) a change of control and (ii) a "ratings decline" (as defined in the indenture). A change of control includes: (i) the sale, lease or transfer of all or substantially all of our assets; (ii) certain ownership changes; (iii) certain recapitalizations, mergers and dispositions; and (iv) stockholder approval of any plan or proposal for the liquidation or dissolution of us. We may also be required to offer to repurchase the 4.625% Senior Notes at 100% of the principal amount, plus any accrued and unpaid interest, with the net proceeds of certain asset sales. 5.875% Senior Notes. In May 2016, we issued $375.0 million aggregate principal amount of 5.875% unsecured senior notes due May 2024 ("5.875% Senior Notes") at 100% of the principal amount. On December 18, 2019, we redeemed in full the $375.0 million aggregate principal balance of our 5.875% Senior Notes at a redemption price of 104.406% of the principal amount plus $2.0 million of accrued interest for a total net cash outflow of $393.5 million. Upon redemption of the 5.875% Senior Notes, we recorded a loss on extinguishment of debt of $20.3 million within Other expense, net on our Statements of Consolidated Income, which included the premium payment of $16.5 million and a write-off of the remaining unamortized debt issuance costs of $3.8 million (see Note 12 for details). The effective interest rate of the 5.875% Senior Notes was approximately 6.1% per annum, taking into account the amortization of debt issuance costs. Interest expense, including amortization of debt issuance costs, relating to the 5.875% Senior Notes was $22.0 million for the year ended December 31, 2019 and $22.9 million for each of the years ended December 31, 2018 and December 31, 2017. A portion of the interest relating to the 5.875% Senior Notes was capitalized as construction in progress. At December 31, 2018, the fair value of the outstanding 5.875% Senior Notes, which were Level 1 liabilities calculated based on trading prices around the balance sheet date, was approximately $369.9 million. Revolving Credit Facility In October 2019, we entered into a revolving credit facility with Wells Fargo Bank, National Association, as administrative agent, and the other financial institutions party thereto ("Revolving Credit Facility"). The Revolving Credit Facility provides us with a $375.0 million funding commitment through October 30, 2024. Joining us as borrowers ("Co-Borrowers") under the Revolving Credit Facility are three of our wholly owned domestic operating subsidiaries: Kaiser Aluminum Investments Company, Kaiser Aluminum Fabricated Products, LLC and Kaiser Aluminum Washington, LLC. The Revolving Credit Facility replaces the previously existing facility that was due to mature in December 2020. The Revolving Credit Facility is secured by a first priority lien on substantially all of the Co-borrowers' accounts receivable, inventory, cash or cash equivalents and certain other related assets and proceeds, as well as certain machinery and equipment. Under the Revolving Credit Facility, we are able to borrow from time to time an aggregate amount equal to the lesser of $375.0 million and a borrowing base comprised of: (i) 90% of eligible accounts receivable in which the account debtor is an investment-grade domestic account debtor; (ii) 85% of eligible accounts receivable in which the account debtor is a domestic account debtor, but not an investment-grade domestic account debtor; (iii) the lesser of (a) 85% of eligible accounts receivable in which the account debtor is not a domestic account debtor and (b) an amount equal to 25% of the lesser of (a) the maximum revolver amount or (b) the borrowing base; (iv) the lesser of (a) the product of 75% multiplied by the value of eligible inventory and (b) the product of 85% multiplied by the net recovery percentage identified in the most recent acceptable appraisal of inventory, multiplied by the value of eligible inventory; (v) certain eligible machinery and equipment supporting up to $71.5 million of borrowing availability; and (vi) at our option, 100% of eligible cash, reduced by certain reserves, all as specified in the Revolving Credit Facility. Up to a maximum of $20.0 million of availability under the Revolving Credit Facility may be utilized for letters of credit. At maturity, all principal amounts outstanding under the Revolving Credit Facility will be due and payable. Borrowings under the Revolving Credit Facility bear interest at a rate equal to either a base prime rate or LIBOR, plus, in each case, a specified variable percentage determined by reference to the then-remaining borrowing availability under the Revolving Credit Facility. The initial LIBOR applicable margin is 125 basis points. We will also pay a monthly commitment fee equal to 0.25% per annum multiplied by the result of the aggregate amount of revolver commitments, less the average revolver usage during the immediately preceding month. The Revolving Credit Facility may, subject to certain conditions and the agreement of lenders thereunder, be increased up to $575.0 million, subject to certain conditions and the agreement of lenders thereunder. We had $360.9 million of total borrowing availability under the Revolving Credit Facility at December 31, 2019 based on the borrowing base determination then in effect. At December 31, 2019, there were no borrowings under the Revolving Credit Facility and $8.0 million was used to support outstanding letters of credit, leaving $352.9 million of net borrowing availability. The unamortized amount of debt issuance costs relating to the Revolving Credit Facility as of December 31, 2019 was $1.5 million. The interest rate applicable to any overnight borrowings under the Revolving Credit Facility would have been 5.00% at December 31, 2019. Amounts owed under the Revolving Credit Facility may be accelerated upon the occurrence of various events of default, including, without limitation, the failure to make principal or interest payments when due and breaches of covenants, representations and warranties set forth therein. The Revolving Credit Facility places limitations on our ability and the ability of certain of our subsidiaries to, among other things, grant liens, engage in mergers, sell assets, incur debt, enter into sale and leaseback transactions, make investments, undertake transactions with affiliates, prepay certain debt, pay dividends and repurchase shares. We are allowed to prepay debt, pay dividends and repurchase shares in any amount if, in accordance with terms as defined in the Revolving Credit Facility, no "default" or "event of default" has occurred and is continuing or would result after giving effect to such action and, after giving effect to such proposed action, "excess availability" calculated on a pro forma basis is greater than a defined minimum level. A lower minimum level applies if the "fixed charge coverage ratio," calculated on a pro forma basis after giving effect to such proposed action for the period of four consecutive fiscal quarters ending on the last day of the most recently ended fiscal quarter for which financial statements have been delivered, is greater than 1.10:1.0. In addition, we are required to maintain a fixed charge coverage ratio on a consolidated basis at or above 1.0:1.0 if borrowing availability under the Revolving Credit Facility is less than a defined minimum level. At December 31, 2019, we were in compliance with all covenants contained in the Revolving Credit Facility.