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Employee Benefit Plans Employee Stock Purchase Plan (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2014
Postemployment Benefits [Abstract] |  |  |  |
Stock Issued During Period, Shares, Employee Stock Purchase Plans | 56,866 | 38,464 | 33,300 |
Weighted Average Fair Value Of Stock Purchase Right Under Employee Stock Purchase Plan | $ 90.63 | $ 133.54 | $ 134.31 |
Aggregate Cost Of Discount Off Market Value For Shares Available Under Employee Stock Purchase Plan | $ 14.10 | $ 23.51 | $ 16.79 |
Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date |  |  | 15.00% | 10.00%

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- Definition This element represents the value of the discount applied to the market value for shares available to participating employees under the employee stock purchase plan. + References No definition available. + Details
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Namespace Prefix: | algt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition This element represents the weighted average fair value of each purchase right under the employee stock purchase plan. + References No definition available. + Details
Name: | algt_WeightedAverageFairValueOfStockPurchaseRightUnderEmployeeStockPurchasePlan
Namespace Prefix: | algt_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

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- References No definition available. + Details
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Period Type: | duration

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- Definition Discount rate from fair value on purchase date that participants pay for shares. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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X
- Definition Number of shares issued during the period as a result of an employee stock purchase plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770 Reference 3: 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(28)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 4: 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(29)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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