Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNonmonetaryNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
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_DerivativeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage of expected requirements covered by the aggregate notional amount of derivative contracts expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract as a percentage of expected need.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=kalu_AlloyMetalHedgeMember
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_PositionAxis=us-gaap_LongMember
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_StatementScenarioAxis=us-gaap_ScenarioForecastMember
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_PositionAxis=us-gaap_ShortMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=kalu_ElectricityMember
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_DerivativeInstrumentRiskAxis=kalu_MidwestPremiumSwapContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Derivatives, Hedging Programs and Other Financial Instruments, Material Derivative Positions (Details) mmlbs in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2018 EUR (€) MMBTU MWH mmlbs
Alloy Metal Hedge \| Designated as Hedging Instrument \| Purchase
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount				6.1
Natural Gas Member \| Designated as Hedging Instrument \| Purchase
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount \| MMBTU				3,520,000
Natural Gas Member \| Scenario, Forecast
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Percent of Required Need, Coverage	69.00%	70.00%	72.00%
Aluminum Member \| Designated as Hedging Instrument \| Purchase
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount				141.0
Aluminum Member \| Designated as Hedging Instrument \| Sales
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount				3.9
Electricity Member \| Designated as Hedging Instrument \| Purchase
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount \| MWH				175,680
Electricity Member \| Scenario, Forecast
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Percent of Required Need, Coverage	53.00%	53.00%	53.00%
Foreign Exchange Contract \| Designated as Hedging Instrument \| Purchase
Derivative [Line Items]
Derivative, Notional Amount \| €				€ 64,555.3
Midwest premium swap contracts \| Designated as Hedging Instrument \| Purchase
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount				136.7