Consolidated net debt ratio.

No definition available.

Name:	kalu_ConsolidatedNetDebtRatio
Namespace Prefix:	kalu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Redemption Price, Percentage Of Principal Amount That Can be Redeemed

+ References

No definition available.

+ Details

Name:	kalu_DebtInstrumentRedemptionPricePercentageOfPrincipalAmountThatCanbeRedeemed
Namespace Prefix:	kalu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt issuance, percentage of principal amount

+ References

No definition available.

+ Details

Name:	kalu_DebtIssuancePercentageOfPrincipalAmount
Namespace Prefix:	kalu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Borrowing Base, Percentage of Eligible Cash

+ References

No definition available.

+ Details

Name:	kalu_LineofCreditFacilityBorrowingBasePercentageofEligibleCash
Namespace Prefix:	kalu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Possible Future Maximum Borrowing Capacity

+ References

No definition available.

+ Details

Name:	kalu_LineofCreditFacilityPossibleFutureMaximumBorrowingCapacity
Namespace Prefix:	kalu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line of Credit, Fixed Charge Coverage Ratio

+ References

No definition available.

+ Details

Name:	kalu_LineofCreditFixedChargeCoverageRatio
Namespace Prefix:	kalu_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum amount of eligible machinery and equipment for borrowing base

+ References

No definition available.

+ Details

Name:	kalu_Maximumamountofeligiblemachineryandequipmentforborrowingbase
Namespace Prefix:	kalu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage of Eligible Accounts Receivable for Investment-grade Borrowing Base

+ References

No definition available.

+ Details

Name:	kalu_PercentageofEligibleAccountsReceivableforInvestmentgradeBorrowingBase
Namespace Prefix:	kalu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of Eligible Accounts Receivable for Non-Domestic Account Debtor

+ References

No definition available.

+ Details

Name:	kalu_PercentageofEligibleAccountsReceivableforNonDomesticAccountDebtor
Namespace Prefix:	kalu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of Maximum Revolver Amount or Borrowing Base

+ References

No definition available.

+ Details

Name:	kalu_PercentageofMaximumRevolverAmountorBorrowingBase
Namespace Prefix:	kalu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of eligible accounts receivable for borrowing base

+ References

No definition available.

+ Details

Name:	kalu_Percentageofeligibleaccountsreceivableforborrowingbase
Namespace Prefix:	kalu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of eligible inventory for borrowing base

+ References

No definition available.

+ Details

Name:	kalu_Percentageofeligibleinventoryforborrowingbase
Namespace Prefix:	kalu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of net orderly liquidation value of eligible inventory

+ References

No definition available.

+ Details

Name:	kalu_Percentageofnetorderlyliquidationvalueofeligibleinventory
Namespace Prefix:	kalu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Date the debt agreement requires the first payment to be made, in CCYY-MM-DD format.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

Name:	us-gaap_DebtInstrumentDateOfFirstRequiredPayment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

+ Details

Name:	us-gaap_DebtInstrumentFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Description of the frequency of periodic payments (monthly, quarterly, annual).

+ References

+ Details

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

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6036836-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateEffectivePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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

- Definition

Description of the payment terms of the debt instrument (for example, whether periodic payments include principal and frequency of payments) and discussion about any contingencies associated with the payment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(3))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

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

- Definition

Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph e
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

Name:	us-gaap_DebtInstrumentRedemptionPricePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate amount of interest paid or due on all long-term debt.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.8)
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

+ Details

Name:	us-gaap_InterestExpenseLongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityCommitmentFeePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

+ Details

Name:	us-gaap_UnamortizedDebtIssuanceExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	kalu_DebtCovenantTriggerAxis=kalu_PaymentConditionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	kalu_DebtCovenantTriggerAxis=kalu_CovenanttriggereventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=us-gaap_LetterOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=kalu_SeniorNotesDueTwoThousandAndTwentyFiveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=kalu_OnOrAfterMayOneTwoThousandTwentyTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=kalu_OnOrAfterMayOneTwoThousandTwentyThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=kalu_OnOrAfterMayOneTwoThousandTwentyFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=kalu_PriorToMayOneTwoThousandTwentyTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=kalu_SeniorNotesDueTwoThousandAndTwentyEightMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	kalu_AccountingChangeAxis=kalu_ChangesInControlMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=kalu_OnOrAfterMarchOneTwoThousandTwentyThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=kalu_OnOrAfterMarchOneTwoThousandTwentyFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=kalu_OnOrAfterMarchOneTwoThousandTwentyFiveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=kalu_PriorToMarchOneTwoThousandTwentyThreePlusApplicablePremiumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=kalu_PriorToMarchOneTwoThousandTwentyThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=kalu_UponReceiptOfProceedsOfCertainAssetSaleMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=kalu_SeniorNotesDueTwoThousandAndTwentyFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt and Credit Facility - Additional Information (Details) - USD ($)		1 Months Ended	12 Months Ended
	Dec. 18, 2019	Oct. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	May 01, 2020	Apr. 28, 2020	Nov. 26, 2019	May 15, 2016
Revolving Credit Facility
Debt Instrument
Line of credit facility, maximum borrowing capacity		$ 375,000,000.0	$ 375,000,000.0
Percentage of eligible accounts receivable in which the account debtor is an investment-grade domestic account debtor (percent)		90.00%
Percentage of eligible accounts receivable for borrowing base		85.00%
Percentage of eligible accounts receivable in which the account debtor is not a domestic account debtor (percent)		85.00%
Percentage of lesser of maximum revolver amount or borrowing base (percent)		25.00%
Percentage of eligible inventory for borrowing base		75.00%
Percentage of net orderly liquidation value of eligible inventory		85.00%
Maximum amount of eligible machinery and equipment for borrowing base		$ 71,500,000
Percentage of eligible cash (percent)		100.00%
Monthly commitment fee (percent)		0.25%
Maximum borrowing capacity under credit facility		$ 575,000,000.0
Revolving Credit Facility \| Payment Condition
Debt Instrument
Fixed charge coverage ratio		1.10
Revolving Credit Facility \| Covenant Trigger Event
Debt Instrument
Fixed charge coverage ratio		1.0
Revolving Credit Facility \| LIBOR
Debt Instrument
Applicable margin, basis points		1.25%
Revolving Credit Facility \| Letter of Credit
Debt Instrument
Line of credit facility, maximum borrowing capacity		$ 20,000,000.0
Senior Notes Due 2025 \| Senior Notes
Debt Instrument
Debt instrument contractual rate (percent)						6.50%	6.50%
Debt issuance, percentage of principal amount						101.00%	100.00%
Debt principal amount			350,000,000.0			$ 50,000,000.0	$ 300,000,000.0
Unamortized debt issuance costs			5,400,000
Interest expense			$ 16,200,000
Effective interest rate (percent)			6.80%
Debt instrument, payment terms			Interest accrues on the 6.50% Senior Notes beginning April 28, 2020 at a rate of 6.50% per annum and is payable semiannually on May 1 and November 1 of each year. The first interest payment date was November 1, 2020
Debt instrument, frequency of interest payment			semiannually
Debt Instrument, first interest payment date			Nov. 01, 2020
Fair value of outstanding debt			$ 376,800,000
Senior Notes Due 2025 \| Senior Notes \| On or After May 1, 2022
Debt Instrument
Redemption price as percentage of principal (percent)			103.25%
Senior Notes Due 2025 \| Senior Notes \| On or After May 1, 2023
Debt Instrument
Redemption price as percentage of principal (percent)			101.625%
Senior Notes Due 2025 \| Senior Notes \| On or After May 1, 2024
Debt Instrument
Redemption price as percentage of principal (percent)			100.00%
Senior Notes Due 2025 \| Senior Notes \| Prior to May 1, 2022
Debt Instrument
Redemption price as percentage of principal (percent)			106.50%
Percentage of principal that can be redeemed (percent)			40.00%
Senior Notes Due 2028 \| Senior Notes
Debt Instrument
Debt instrument contractual rate (percent)								4.625%
Debt issuance, percentage of principal amount								100.00%
Debt principal amount								$ 500,000,000.0
Unamortized debt issuance costs			$ 6,500,000
Interest expense			$ 24,000,000.0	$ 2,400,000
Effective interest rate (percent)			4.80%
Fair value of outstanding debt			$ 521,300,000	513,500,000
Consolidated net debt ratio			275.00%
Senior Notes Due 2028 \| Senior Notes \| Change in control
Debt Instrument
Redemption price as percentage of principal (percent)			101.00%
Senior Notes Due 2028 \| Senior Notes \| On or After March 1, 2023
Debt Instrument
Redemption price as percentage of principal (percent)			102.313%
Senior Notes Due 2028 \| Senior Notes \| On or After March 1, 2024
Debt Instrument
Redemption price as percentage of principal (percent)			101.156%
Senior Notes Due 2028 \| Senior Notes \| On or After March 1, 2025
Debt Instrument
Redemption price as percentage of principal (percent)			100.00%
Senior Notes Due 2028 \| Senior Notes \| Prior to March 1, 2023, Plus Applicable Premium
Debt Instrument
Redemption price as percentage of principal (percent)			100.00%
Senior Notes Due 2028 \| Senior Notes \| Prior to March 1, 2023
Debt Instrument
Redemption price as percentage of principal (percent)			104.625%
Senior Notes Due 2028 \| Senior Notes \| Prior to March 1, 2023 \| Maximum
Debt Instrument
Percentage of principal that can be redeemed (percent)			40.00%
Senior Notes Due 2028 \| Senior Notes \| Upon receipt of the proceeds of certain asset sale
Debt Instrument
Redemption price as percentage of principal (percent)			100.00%
Senior Notes Due 2024 \| Senior Notes
Debt Instrument
Debt instrument contractual rate (percent)									5.875%
Debt issuance, percentage of principal amount									100.00%
Debt principal amount									$ 375,000,000.0
Interest expense				$ 22,000,000.0	$ 22,900,000
Effective interest rate (percent)				6.10%
Redemption price as percentage of principal (percent)	104.406%