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EMPLOYEE BENEFIT PLANS EMPLOYEE BENEFIT PLANS - ADDITIONAL INFORMATION (Details) | 12 Months Ended
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Dec. 31, 2020 plan
Business Acquisition [Line Items] |
Number of Qualified Defined Benefit Plans | 4
Number of defined benefit plans | 1
Number of defined contribution plans | 7

X
- Definition Number of Defined Contribution Plans + References No definition available. + Details
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Namespace Prefix: | alk_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of Non-Qualified Defined Benefit Plans + References No definition available. + Details
Name: | alk_NumberofNonQualifiedDefinedBenefitPlans
Namespace Prefix: | alk_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of Qualified Defined Benefit Plans + References No definition available. + Details
Name: | alk_NumberofQualifiedDefinedBenefitPlans
Namespace Prefix: | alk_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- 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_BusinessAcquisitionLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration