Business, Product, and Geographical Area Information - Schedule of Significant Segment Expenses Provided to CODM (Parenthetical) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2025	Mar. 31, 2024
Segment Reporting [Abstract]
Hedged cost realized gains (losses) upon settlement	$ 4.6	$ (3.5)

Hedged cost realized gains (losses) upon settlement.

No definition available.

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