Commitments and Contingencies, Environmental (Details) $ in Millions	3 Months Ended
Mar. 31, 2018 USD ($)
Environmental Contingency
Environmental accrual	$ 16.6
Expected period related to remediation expenditures for environmental contingencies period	30 years
Potential increase in environmental costs	$ 12.2
Time period within which Companys recorded estimate of its obligation may change	12 months
Minimum
Environmental Contingency
Period for final feasibility study	12 months
Maximum
Environmental Contingency
Period for final feasibility study	18 months

ExpectedPeriodRelatedToRemediationExpendituresForEnvironmentalContingencies

No definition available.

Name:	kalu_ExpectedPeriodRelatedToRemediationExpendituresForEnvironmentalContingenciesPeriod
Namespace Prefix:	kalu_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Period For Final Feasibility Study

+ References

No definition available.

+ Details

Name:	kalu_PeriodForFinalFeasibilityStudy
Namespace Prefix:	kalu_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Time period within which Company's recorded estimate of its obligation may change.

+ References

No definition available.

+ Details

Name:	kalu_TimePeriodWithinWhichCompanysRecordedEstimateOfItsObligationMayChange
Namespace Prefix:	kalu_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Total costs accrued as of the balance sheet date for environmental loss contingencies.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 25
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346

+ Details

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

- Definition

The difference between the maximum amount of estimated loss and the amount recorded as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 05
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6395354&loc=d3e12631-108344

+ Details

Name:	us-gaap_LossContingencyRangeOfPossibleLossPortionNotAccrued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_SiteContingencyLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: