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Employee Benefit Plans Employee Stock Purchase Plan (Details) - USD ($)	12 Months Ended
	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%

X

- 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_
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Balance Type:	debit
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X

- Definition

This element represents the weighted average fair value of each purchase right under the employee stock purchase plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	algt_
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Balance Type:	na
Period Type:	instant

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- References

No definition available.

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X

- 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
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X

- Definition

Number of shares issued during the period as a result of an employee stock purchase plan.

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-Name Accounting Standards Codification
-Topic 505
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