Debt issuance, percentage of principal amount

No definition available.

Name:	kalu_Debtissuancepercentageofprincipalamount
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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit, Ability to Make Restricted Actions, Available Borrowing Necessary After Action

+ References

No definition available.

+ Details

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

- Definition

Line of Credit, Ability to Make Restricted Actions, Available Borrowing Necessary After Action with Fixed Charge Coverage Ratio

+ References

No definition available.

+ Details

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

- Definition

Line of Credit Facility, Possible Future Maximum Borrowing Capacity

+ References

No definition available.

+ Details

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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

Line of Credit, Minimum Borrowing Availability Threshold to Maintain Fixed Charge Coverage Ratio

+ References

No definition available.

+ Details

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

- 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 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

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
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-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=109250915&loc=d3e13433-108611

+ Details

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

- 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)
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-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&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

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

Amount of gain (loss) from the difference between the repurchase price of a debt instrument initially issued by the entity and the net carrying amount of the debt at the time of its repurchase.

+ References

No definition available.

+ Details

Name:	us-gaap_GainLossOnRepurchaseOfDebtInstrument
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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=114873765&loc=SL114874048-224260

+ Details

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

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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+ Details

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

- Definition

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

+ References

+ Details

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

- Definition

The effective interest rate at the end of the reporting period.

+ References

+ Details

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

- 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

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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

- Definition

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

+ Details

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

- Definition

The cash outflow to repay long-term debt that is not secured by collateral. Excludes repayments of tax exempt unsecured debt.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3291-108585

+ Details

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

- 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_DebtInstrumentAxis=kalu_SeniorNotesDue2024Member
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:	kalu_AccountingChangesAxis=kalu_ChangeInControlMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=kalu_SeniorNotesDue2020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=kalu_OnOrAfterJune12016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=kalu_OnorafterMay152019Member
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Period Type:

- Details

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Debt and Credit Facility, Narrative (Details)	6 Months Ended	12 Months Ended
Debt and Credit Facility, Narrative (Details)	Jun. 30, 2016	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 01, 2016 USD ($)	May 15, 2016 USD ($)	May 23, 2012 USD ($)
Debt Instrument
Loss on Repurchase of Debt Instrument		$ 0	$ 0	$ (11,100,000)
Long-term debt		370,400,000	369,600,000
Repayments of Unsecured Debt		0	0	206,000,000.0
Senior Notes Due 2024 \| Senior Notes
Debt Instrument
Principal amount of notes						$ 375,000,000.0
Interest rate						5.875%
Percentage of principal amount issued						100.00%
Unamortized Debt Issuance Expense		4,600,000	5,400,000
Interest Expense including amortization of deferred financing cost		$ 22,900,000		14,500,000
Effective interest rate		6.10%
Debt Instrument, Fair Value Disclosure		$ 369,900,000	$ 399,900,000
Senior Notes Due 2024 \| Senior Notes \| Change in control
Debt Instrument
Debt Instrument, Redemption Price, Percentage		101.00%
Senior Notes Due 2020 \| Senior Notes
Debt Instrument
Principal amount of notes							$ 225,000,000.0
Interest rate							8.25%
Interest Expense including amortization of deferred financing cost				$ 7,100,000
Long-term debt					$ 197,800,000
Revolving Credit Facility \| Line of Credit
Debt Instrument
Maximum borrowing capacity		$ 300,000,000.0
Percentage of eligible accounts receivable for borrowing base		85.00%
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		$ 60,000,000.0
Line of Credit Facility, Possible Future Maximum Borrowing Capacity		400,000,000.0
Available borrowing capacity		300,000,000.0
Outstanding line of credit		0
Remaining available borrowing capacity		$ 292,000,000.0
Interest on Revolving Credit Facility		5.75%
Minimum Amount of Available Borrowing to Maintain		$ 52,500,000
Revolving Credit Facility \| Line of Credit \| Borrowing Availability Under $45.0 Million
Debt Instrument
Line of Credit, Ability to Make Restricted Actions, Available Borrowing Necessary After Action with Fixed Charge Coverage Ratio		$ 45,000,000.0
Fixed Charge Coverage Ratio		1.15
Revolving Credit Facility \| Line of Credit \| Borrowing Availability Under $30.0 Million
Debt Instrument
Fixed Charge Coverage Ratio		1.0
Line of Credit, Minimum Borrowing Availability Threshold to Maintain Fixed Charge Coverage Ratio		$ 30,000,000.0
Revolving Credit Facility \| Letter of Credit
Debt Instrument
Maximum borrowing capacity		20,000,000.0
Outstanding line of credit		$ (8,000,000.0)
On or after June 1, 2016 \| Senior Notes Due 2020 \| Senior Notes
Debt Instrument
Debt Instrument, Redemption Price, Percentage	104.125%
On or after May 15, 2019 \| Senior Notes Due 2024 \| Senior Notes
Debt Instrument
Debt Instrument, Redemption Price, Percentage		104.406%
On or after May 15, 2020 \| Senior Notes Due 2024 \| Senior Notes
Debt Instrument
Debt Instrument, Redemption Price, Percentage		102.938%
On or after May 15, 2021 \| Senior Notes Due 2024 \| Senior Notes
Debt Instrument
Debt Instrument, Redemption Price, Percentage		101.469%
On or after May 15, 2022 \| Senior Notes Due 2024 \| Senior Notes
Debt Instrument
Debt Instrument, Redemption Price, Percentage		100.00%
Any time prior to May 15, 2019 \| Senior Notes Due 2024 \| Senior Notes
Debt Instrument
Debt Instrument, Redemption Price, Percentage		100.00%
Upon receipt of the proceeds of certain asset sales \| Senior Notes Due 2024 \| Senior Notes
Debt Instrument
Debt Instrument, Redemption Price, Percentage		100.00%