Fair Value Measurements - Assets Measured at Fair Value On a Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-Sale	$ 750,461	$ 707,109
Cash equivalents
Cash Equivalents	84,679	160,150
Short-term
Debt Securities, Available-for-Sale	632,959	495,234
Long-term
Debt Securities, Available-for-Sale	32,823	51,725
Level 1
Debt Securities, Available-for-Sale	42,833	41,494
Level 1 \| Cash equivalents
Cash Equivalents	42,833	41,494
Level 1 \| Short-term
Debt Securities, Available-for-Sale	0	0
Level 1 \| Long-term
Debt Securities, Available-for-Sale	0	0
Level 2
Debt Securities, Available-for-Sale	707,628	665,615
Level 2 \| Cash equivalents
Cash Equivalents	41,846	118,656
Level 2 \| Short-term
Debt Securities, Available-for-Sale	632,959	495,234
Level 2 \| Long-term
Debt Securities, Available-for-Sale	32,823	51,725
US Government and agency obligations \| Cash equivalents
Cash Equivalents	16,901	81,535
US Government and agency obligations \| Short-term
Debt Securities, Available-for-Sale	67,696	94,295
US Government and agency obligations \| Long-term
Debt Securities, Available-for-Sale	2,696	10,452
US Government and agency obligations \| Level 1 \| Cash equivalents
Cash Equivalents	0	0
US Government and agency obligations \| Level 1 \| Short-term
Debt Securities, Available-for-Sale	0	0
US Government and agency obligations \| Level 1 \| Long-term
Debt Securities, Available-for-Sale	0	0
US Government and agency obligations \| Level 2 \| Cash equivalents
Cash Equivalents	16,901	81,535
US Government and agency obligations \| Level 2 \| Short-term
Debt Securities, Available-for-Sale	67,696	94,295
US Government and agency obligations \| Level 2 \| Long-term
Debt Securities, Available-for-Sale	2,696	10,452
Money market funds \| Cash equivalents
Cash Equivalents	42,833	41,494
Money market funds \| Level 1 \| Cash equivalents
Cash Equivalents	42,833	41,494
Money market funds \| Level 2 \| Cash equivalents
Cash Equivalents	0	0
Commercial paper \| Cash equivalents
Cash Equivalents	14,712	22,689
Commercial paper \| Short-term
Debt Securities, Available-for-Sale	179,697	149,807
Commercial paper \| Level 1 \| Cash equivalents
Cash Equivalents	0	0
Commercial paper \| Level 1 \| Short-term
Debt Securities, Available-for-Sale	0	0
Commercial paper \| Level 2 \| Cash equivalents
Cash Equivalents	14,712	22,689
Commercial paper \| Level 2 \| Short-term
Debt Securities, Available-for-Sale	179,697	149,807
Certificates of deposit \| Short-term
Debt Securities, Available-for-Sale	27,960	7,239
Certificates of deposit \| Level 1 \| Short-term
Debt Securities, Available-for-Sale	0	0
Certificates of deposit \| Level 2 \| Short-term
Debt Securities, Available-for-Sale	27,960	7,239
Corporate debt securities \| Cash equivalents
Cash Equivalents	5,713	4,133
Corporate debt securities \| Short-term
Debt Securities, Available-for-Sale	337,988	242,313
Corporate debt securities \| Long-term
Debt Securities, Available-for-Sale	30,127	39,931
Corporate debt securities \| Level 1 \| Cash equivalents
Cash Equivalents	0	0
Corporate debt securities \| Level 1 \| Short-term
Debt Securities, Available-for-Sale	0	0
Corporate debt securities \| Level 1 \| Long-term
Debt Securities, Available-for-Sale	0	0
Corporate debt securities \| Level 2 \| Cash equivalents
Cash Equivalents	5,713	4,133
Corporate debt securities \| Level 2 \| Short-term
Debt Securities, Available-for-Sale	337,988	242,313
Corporate debt securities \| Level 2 \| Long-term
Debt Securities, Available-for-Sale	30,127	39,931
Municipal debt securities \| Cash equivalents
Cash Equivalents	4,520	10,299
Municipal debt securities \| Short-term
Debt Securities, Available-for-Sale	19,618	1,580
Municipal debt securities \| Long-term
Debt Securities, Available-for-Sale	0	1,342
Municipal debt securities \| Level 1 \| Cash equivalents
Cash Equivalents	0	0
Municipal debt securities \| Level 1 \| Short-term
Debt Securities, Available-for-Sale	0	0
Municipal debt securities \| Level 1 \| Long-term
Debt Securities, Available-for-Sale	0	0
Municipal debt securities \| Level 2 \| Cash equivalents
Cash Equivalents	4,520	10,299
Municipal debt securities \| Level 2 \| Short-term
Debt Securities, Available-for-Sale	19,618	1,580
Municipal debt securities \| Level 2 \| Long-term
Debt Securities, Available-for-Sale	$ 0	$ 1,342

Allegiant Travel CO 10-K