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Summary of Significant Accounting Policies Interest Costs Capitalized (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Interest Cost [Abstract] |  |  |
Capitalized interest | $ 4,067 | $ 4,472 | $ 2,354

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- Definition Interest Cost [Abstract] + References No definition available. + Details
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Namespace Prefix: | algt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of interest costs capitalized disclosed as an adjusting item to interest costs incurred. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
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