Commitments and Contingencies - Summary of Activities Relating to Company's CARO Liabilities (Parenthetical) (Details) - $ / shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Adjustment to accretion expense, impact on basic and diluted net income per share	$ 0.01	$ 0.01

Adjustment to accretion expense impact on earnings per share basic and diluted.

No definition available.

Name:	kalu_AdjustmentToAccretionExpenseImpactOnEarningsPerShareBasicAndDiluted
Namespace Prefix:	kalu_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration