Fresh Start Accounting - Schedule of Consolidated Successor Balance Sheet (Details) - USD ($) $ / shares in Units, $ in Thousands	Jun. 30, 2025	Mar. 31, 2025	Mar. 13, 2025	Mar. 12, 2025	Dec. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Current assets:
Cash and cash equivalents	$ 407,511		$ 388,607		$ 902,057
Restricted cash			176,618
Short-term investment securities			119,315
Accounts receivable, net			201,681
Prepaid expenses and other current assets	253,057		257,294		278,366
Assets held for sale	449,149		447,271		463,020
Total current assets	1,481,219		1,590,787		2,109,122
Property and equipment:
Flight equipment	1,893,585		1,888,698		2,736,461
Ground property and equipment	447,030		441,866		783,645
Less accumulated depreciation	(51,434)		0		(1,027,872)
Total property and equipment, net	2,289,181		2,330,564		2,492,234
Operating lease right-of-use assets	4,457,889		4,436,918		4,583,734
Intangible assets	83,482		83,482		550
Pre-delivery deposits on flight equipment	73,572		85,495		113,493
Deferred heavy maintenance, net	115,451		125,705		241,094
Other long-term assets	75,493		67,043		54,951
Total assets	8,576,287		8,719,994	$ 8,720,000	9,595,178
Current liabilities:
Accounts payable	147,676		46,676		32,385
Air traffic liability	407,473		518,668		436,813
Current maturities of long-term debt, net, and finance leases	121,190		165,689		436,532
Current maturities of operating leases	239,633		242,230		257,796
Other current liabilities	537,556		582,249		605,839
Total current liabilities	1,453,528		1,555,512		1,769,365
Long-term debt, net and finance leases, less current maturities	2,242,448		2,054,124		1,761,215
Operating leases, less current maturities	4,228,832		4,208,781		4,335,106
Deferred income taxes	64,757		69,408		51,927
Deferred gains and other long-term liabilities	107,277		97,799		122,595
Total liabilities not subject to compromise			7,985,623
Liabilities subject to compromise	0			1,600,000	1,635,104
Shareholders’ equity:
Predecessor common stock	2		2		11
Predecessor Additional paid-in capital	736,212		734,368		1,173,692
Predecessor Treasury stock at cost	0				(81,285)
Retained Earnings (Accumulated Deficit)	(256,767)				(1,172,740)
Accumulated other comprehensive income (loss)	(2)				188
Total shareholders’ equity (deficit)	479,445	$ 723,413	734,370	$ 734,370	(80,134)	$ 809,662	$ 1,002,257	$ 1,134,342
Total liabilities and shareholders’ equity (deficit)	$ 8,576,287		8,719,994		$ 9,595,178
Common stock, par value (in dollars per share)	$ 0.0001			$ 0.0001
Predecessor
Current assets:
Cash and cash equivalents				$ 678,382
Restricted cash				171,325
Short-term investment securities				119,315
Accounts receivable, net				201,681
Prepaid expenses and other current assets				259,522
Assets held for sale				447,271
Total current assets				1,877,498
Property and equipment:
Flight equipment				2,739,143
Ground property and equipment				787,057
Less accumulated depreciation				(1,062,116)
Total property and equipment, net				2,464,084
Operating lease right-of-use assets				4,631,428
Intangible assets				550
Pre-delivery deposits on flight equipment				85,495
Deferred heavy maintenance, net				246,576
Other long-term assets				67,043
Total assets				9,372,673
Current liabilities:
Accounts payable				52,242
Air traffic liability				518,668
Current maturities of long-term debt, net, and finance leases				471,698
Current maturities of operating leases				259,713
Other current liabilities				623,035
Total current liabilities				1,925,357
Long-term debt, net and finance leases, less current maturities				1,704,517
Operating leases, less current maturities				4,380,845
Deferred income taxes				52,556
Deferred gains and other long-term liabilities				120,795
Total liabilities not subject to compromise				8,184,070
Liabilities subject to compromise				1,635,104
Shareholders’ equity:
Predecessor common stock				11
Predecessor Additional paid-in capital				1,174,925
Predecessor Treasury stock at cost				(81,285)
Retained Earnings (Accumulated Deficit)				(1,540,278)
Accumulated other comprehensive income (loss)				127
Total shareholders’ equity (deficit)				(446,501)
Total liabilities and shareholders’ equity (deficit)				9,372,673
Reorganization Items
Current assets:
Cash and cash equivalents				(289,775)
Restricted cash				5,293
Prepaid expenses and other current assets				(2,229)
Total current assets				(286,711)
Property and equipment:
Total assets				(286,711)
Current liabilities:
Accounts payable				(5,566)
Current maturities of long-term debt, net, and finance leases				(309,000)
Other current liabilities				(39,250)
Total current liabilities				(353,816)
Long-term debt, net and finance leases, less current maturities				526,841
Deferred income taxes				16,900
Total liabilities not subject to compromise				173,025
Liabilities subject to compromise				(1,635,104)
Shareholders’ equity:
Predecessor common stock			2	(11)
Predecessor Additional paid-in capital			$ 734,368	(1,174,925)
Predecessor Treasury stock at cost				81,285
Retained Earnings (Accumulated Deficit)				1,534,648
Total shareholders’ equity (deficit)				1,175,368
Total liabilities and shareholders’ equity (deficit)				(286,711)
Fresh Start Adjustment
Current assets:
Total current assets				0
Property and equipment:
Flight equipment				(850,445)
Ground property and equipment				(345,190)
Less accumulated depreciation				1,062,116
Total property and equipment, net				(133,520)
Operating lease right-of-use assets				(194,510)
Intangible assets				82,932
Deferred heavy maintenance, net				(120,871)
Total assets				(365,969)
Current liabilities:
Current maturities of long-term debt, net, and finance leases				2,991
Current maturities of operating leases				(17,483)
Other current liabilities				(1,536)
Total current liabilities				(16,029)
Long-term debt, net and finance leases, less current maturities				(177,234)
Operating leases, less current maturities				(172,065)
Deferred income taxes				16,852
Deferred gains and other long-term liabilities				(22,996)
Total liabilities not subject to compromise				(371,472)
Shareholders’ equity:
Retained Earnings (Accumulated Deficit)				5,630
Accumulated other comprehensive income (loss)				(127)
Total shareholders’ equity (deficit)				5,503
Total liabilities and shareholders’ equity (deficit)				$ (365,969)