Business, Product, and Geographical Area Information - Schedule of Significant Segment Expenses Provided to CODM (Details) - USD ($)
$ in Millions

3 Months Ended

9 Months Ended

Sep. 30, 2025

Jun. 30, 2025

Mar. 31, 2025

Sep. 30, 2024

Jun. 30, 2024

^[11]

Mar. 31, 2024

^[11]

Sep. 30, 2025

Sep. 30, 2024

Segment Reporting Information [Line Items]

Net sales

$ 843.5

$ 747.7

^[1],[2]

$ 2,444.0

$ 2,258.6

^[1],[2]

Hedged cost of alloyed metal

^[3]

492.8

385.7

^[2]

1,355.9

1,160.8

^[2]

Manufacturing costs

^[4]

158.8

208.2

^[2]

532.5

599.1

^[2]

Plant overhead

^[5]

45.9

45.0

^[2]

136.0

133.5

^[2]

Freight costs

21.6

23.4

^[2]

63.6

69.1

^[2]

Other cost of products sold

^[6]

9.7

13.7

^[2]

37.0

34.6

^[2]

Depreciation and amortization

32.0

29.0

^[1],[2]

91.6

86.8

^[1],[2]

Selling, general, administrative, research and development

33.9

28.8

^[1]

97.3

93.0

^[1]

Research and development costs

0.4

0.5

^[2]

1.0

1.7

^[2]

Employee costs

^[7]

24.7

20.2

^[2]

69.9

63.8

^[2]

Other selling, general and administrative costs

^[8]

8.8

8.1

^[2]

26.4

27.5

^[2]

Restructuring costs

0.0

0.7

^[2]

1.9

7.6

^[2]

Other operating charges, net

0.0

^[1],[2]

0.0

0.4

^[1],[2]

Interest expense

12.4

10.7

^[1],[2]

36.1

33.3

^[1],[2]

Other income, net - Note 9

(11.4)

(8.7)

^[1],[2]

(14.4)

(19.1)

^[1],[2]

Income tax provision

8.3

2.4

^[1],[9]

22.2

13.6

^[1],[9]

Net income

$ 39.5

$ 23.2

$ 21.6

$ 8.8

^[1],[10]

$ 18.9

$ 18.2

$ 84.3

^[12]

$ 45.9

^{[1],[10],[12]}

[1]	Adjusted to reflect the retrospective change in inventory valuation methodology from LIFO to WAC. See Note 14 for further discussion.
[2]	Adjusted to reflect the retrospective change in inventory valuation methodology from LIFO to WAC. See Note 14 for further discussion.
[3]	Hedged cost of alloyed metal includes cost of aluminum at the MWTP and the cost of alloying elements used in the production process. For the quarters ended September 30, 2025 and September 30, 2024, this metric also includes metal price exposure on shipments that we hedged with realized gains (losses) upon settlement of $10.2 million and ($2.1) million, respectively. For the nine months ended September 30, 2025 and September 30, 2024, this metric also includes metal price exposure on shipments that we hedged with realized gains (losses) upon settlement of $15.4 million and ($2.2) million, respectively.
[4]	Manufacturing costs primarily includes labor, utilities, supplies, metal valuation impacts, metal profits, and other materials, excluding alloys incurred at our various production facilities.
[5]	Plant overhead includes salaried employee costs, property taxes, and insurance associated with our various production facilities.
[6]	Other cost of products sold primarily includes lease expense, accretion expense related to conditional asset retirement obligations, gains and losses on operating asset disposals, and major maintenance costs.
[7]	Employee costs include non direct labor salaries, benefits, and incentive compensation.
[8]	Other selling, general and administrative costs primarily include professional services, computer hardware and software costs, office rent, and utilities.
[9]	Adjusted to reflect the retrospective change in inventory valuation methodology from LIFO to WAC. See Note 14 for further discussion.
[10]	Adjusted to reflect the retrospective change in inventory valuation methodology from LIFO to WAC. See Note 14 for further discussion.
[11]	Adjusted to reflect the retrospective change in inventory valuation methodology from LIFO to WAC. See Note 14 for further discussion.
[12]	See Note 12 for supplemental cash flow information.

KAISER ALUMINUM CORP 10-Q