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Employee Benefits - Summary of Total Expense Related to All Postretirement Benefit Plans (Parenthetical) (Details) - USD ($) $ in Millions	6 Months Ended	15 Months Ended
	Jun. 30, 2024	Jun. 30, 2025
2024 Plan
Defined Benefit Plan Disclosure [Line Items]
Multiemployer pension plans excludes charge to restructuring costs	$ 4.6	$ 4.6

X

- Definition

Restructuring cost, pension obligation expected

+ References

No definition available.

+ Details

Name:	kalu_RestructuringCostPensionObligationExpected
Namespace Prefix:	kalu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_RestructuringPlanAxis=kalu_TwoThousandAndTwentyFourRestructuringPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: