Reclassification from AOCI due to cancellation of firm commitment.

No definition available.

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

- Definition

Reclassification from AOCI due to forecasted sales no longer probable of occurring.

+ References

No definition available.

+ Details

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

- Definition

Amount of gain (loss) from the increase (decrease) in fair value of derivative and nonderivative instruments designated as fair value hedging instruments recognized in the income statement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 25
-Section 50
-Paragraph 1
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=109977999&loc=d3e76258-113986

+ Details

Name:	us-gaap_ChangeInUnrealizedGainLossOnFairValueHedgingInstruments1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of the increase (decrease) in fair value of the hedged item in a fair value hedge recognized in the income statement.

+ References

+ Details

Name:	us-gaap_ChangeInUnrealizedGainLossOnHedgedItemInFairValueHedge1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2(a),(d))
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

+ Details

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

- Definition

The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

+ Details

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

- Definition

Amount of gain (loss) on derivative instrument designated and qualifying as fair value hedge included in assessment of hedge effectiveness, recognized in earnings.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 20
-Section 45
-Paragraph 1A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109974929&loc=SL110061172-113977

+ Details

Name:	us-gaap_DerivativeFairValueHedgeIncludedInEffectivenessGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959

+ Details

Name:	us-gaap_DerivativeGainLossOnDerivativeNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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

- Definition

Amount, before tax and after adjustments, of gain (loss) from increase (decrease) in value of excluded component of derivative designated and qualifying as hedge, attributable to parent. Adjustments include, but are not limited to, reclassifications for sale and settlement, and amounts recognized under systematic and rational method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (dd)
-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669646-108580

+ Details

Name:	us-gaap_OtherComprehensiveIncomeLossDerivativeExcludedComponentIncreaseDecreaseAfterAdjustmentsBeforeTaxParent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_IncomeStatementLocationAxis=kalu_DepreciationandAmortizationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_CostOfSalesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=us-gaap_ElectricityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=kalu_AluminumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=kalu_AlloyMetalHedgeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=kalu_AlloyMetalReclassificationDueToForecastedTransactionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=kalu_NaturalGasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_FairValueHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Derivatives, Hedging Programs and Other Financial Instruments - Summary of Loss (Gain) Associated with Derivative Contracts (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Derivative Instruments Gain Loss [Line Items]
Total of income and expense line items presented in our Statements of Consolidated (Loss) Income in which the effects of cash flow hedges are recorded	$ 224.5	$ 303.5	$ 511.1	$ 618.6
Total of income and expense line items presented in our Statements of Consolidated (Loss) Income in which the effects of fair value hedges are recorded	13.0	12.1	26.2	24.0
Depreciation and Amortization \| Designated as Hedging Instrument \| Foreign Exchange Contract
Derivative Instruments Gain Loss [Line Items]
Foreign exchange contracts	(0.2)
Cash Flow Hedges \| Cost of Sales \| Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Loss recognized in our Statements of Consolidated (Loss) Income related to cash flow hedges	10.5	4.7	17.5	8.9
Cash Flow Hedges \| Cost of Sales \| Designated as Hedging Instrument \| Electricity
Derivative Instruments Gain Loss [Line Items]
Loss recognized in our Statements of Consolidated (Loss) Income related to cash flow hedges	0.3		0.5
Cash Flow Hedges \| Cost of Sales \| Designated as Hedging Instrument \| Aluminum
Derivative Instruments Gain Loss [Line Items]
Loss recognized in our Statements of Consolidated (Loss) Income related to cash flow hedges	7.6	4.5	13.3	8.7
Cash Flow Hedges \| Cost of Sales \| Designated as Hedging Instrument \| Alloying Metals
Derivative Instruments Gain Loss [Line Items]
Loss recognized in our Statements of Consolidated (Loss) Income related to cash flow hedges	1.5	0.1	2.1	$ 0.2
Cash Flow Hedges \| Cost of Sales \| Designated as Hedging Instrument \| Alloying Metals (Reclassification Due To Forecasted Transactions No Longer Probable of Occuring
Derivative Instruments Gain Loss [Line Items]
Alloying metals (reclassification due to forecasted transactions no longer probable of occurring)	0.7		0.7
Cash Flow Hedges \| Cost of Sales \| Designated as Hedging Instrument \| Natural Gas
Derivative Instruments Gain Loss [Line Items]
Loss recognized in our Statements of Consolidated (Loss) Income related to cash flow hedges	0.4	$ 0.1	0.9
Fair Value Hedges \| Depreciation and Amortization \| Designated as Hedging Instrument \| Foreign Exchange Contract
Derivative Instruments Gain Loss [Line Items]
Loss recognized in our Statements of Consolidated (Loss) Income related to cash flow hedges	0.1		0.3
Hedged item	0.2
Excluded component amortized from AOCI	0.2		0.4
Reclassification from AOCI related to early termination of firm commitment	(0.4)		(0.4)
Net settlement related to early termination of forward contracts	$ 0.3		$ 0.3