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Business, Product, and Geographical Area Information - Schedule of Significant Segment Expenses Provided to CODM (Parenthetical) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting [Abstract]
Hedged cost realized gains (losses) upon settlement	$ 10.2	$ (2.1)	$ 15.4	$ (2.2)

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- Definition

Hedged cost realized gains (losses) upon settlement.

+ References

No definition available.

+ Details

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Namespace Prefix:	kalu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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- References

No definition available.

+ Details

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