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LONG-TERM DEBT - LINE OF CREDIT (Details) | Dec. 31, 2019 USD ($) credit_facility | Sep. 30, 2019 USD ($) | Jun. 30, 2019 USD ($) | Mar. 31, 2019 USD ($)
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Line of Credit Facility [Line Items] |  |  |  |
Line of credit facility, borrowing capacity | $ 516,000,000 |  |  |
Number of credit facilities | credit_facility | 3 |  |  |
Credit Facility 1 [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Line of credit facility, asset restrictions, unrestricted cash and marketable securities | $ 500,000,000 |  |  |
Credit Facility 1 [Member] | Secured by aircraft [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Line of credit facility, borrowing capacity |  |  | $ 250,000,000 |
Number of credit facilities | credit_facility | 1 |  |  |
Credit Facility 2 [Member] | Secured by other [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Line of credit facility, borrowing capacity |  |  |  | $ 150,000,000
Credit Facility 3 [Member] | Secured by aircraft [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Line of credit facility, borrowing capacity |  | $ 116,000,000 |  |
Credit Facilities 1 and 2 [Member] |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |
Number of credit facilities, no borrowing | credit_facility | 2 |  |  |

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- Definition Line of Credit Facility, Asset Restrictions, Unrestricted Cash and Marketable Securities + References No definition available. + Details
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Namespace Prefix: | alk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Line of Credit Facility, Number of Credit Facilities + References No definition available. + Details
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Namespace Prefix: | alk_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition Line of Credit Facility, Number of Credit Facilities, No Borrowing + References No definition available. + Details
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Namespace Prefix: | alk_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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

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- 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 Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Period Type: |

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- Details
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- Details
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- Details
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Balance Type: |
Period Type: |