WORLDWIDE PANDEMIC - Narrative (Details)													2 Months Ended	3 Months Ended				5 Months Ended	6 Months Ended			9 Months Ended	12 Months Ended	14 Months Ended
	Apr. 23, 2026	Jan. 15, 2026	Jun. 03, 2021 USD ($) shares	Apr. 23, 2021 USD ($) shares	Mar. 05, 2021 USD ($) shares	Jan. 15, 2021 USD ($) shares	Sep. 30, 2020 USD ($) shares	Jul. 30, 2020 USD ($) shares	Jun. 30, 2020 USD ($) shares	Jun. 01, 2020 employee	May 29, 2020 USD ($) shares	Apr. 21, 2020 USD ($) shares	Jun. 30, 2021 USD ($) employee supportProgram aircraft $ / shares shares	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($) employee supportProgram aircraft $ / shares shares	Mar. 31, 2021 USD ($)	Jun. 30, 2020 USD ($) shares	Sep. 30, 2020 USD ($) shares	Jun. 30, 2021 USD ($) employee supportProgram aircraft $ / shares shares	Jun. 30, 2021 USD ($) supportProgram aircraft employee $ / shares shares	Jun. 30, 2020 USD ($) shares	Dec. 31, 2020 USD ($) shares	Dec. 31, 2020 USD ($) shares	Jun. 30, 2021 USD ($) employee supportProgram aircraft $ / shares shares
Unusual or Infrequent Item, or Both [Line Items]
Promissory Note															$ 0		$ 2,683,000,000			$ 0	$ 3,683,000,000
Payroll support and voluntary Employee programs, net															740,000,000		$ 784,000,000			2,187,000,000	$ 784,000,000
Deferred Payroll Support Program grant proceeds													$ 763,000,000		$ 763,000,000				$ 763,000,000	$ 763,000,000		$ 0	$ 0	$ 763,000,000
COVID - 19
Unusual or Infrequent Item, or Both [Line Items]
Number of support programs | supportProgram													3		3				3	3				3
Voluntary separation program 2020 participants | employee													4,200		4,200				4,200	4,200				4,200
Number of days before the end of their term										90 days
Employees who elected conversion in extended emergency time off | employee																				300
Number of employees who participated in extended emergency time off during 2020 | employee										11,000			7,000		7,000				7,000	7,000				7,000
Total employees who participated in extended emergency time off During 2020, who were still on emergency time off at December 31, 2020 | employee										1,466
Extended emergency time off reversal for employees who accepted through March 31, 2021															$ 15,000,000	$ 115,000,000
Proceeds from PSP2 payroll support program															724,000,000					$ 1,900,000,000
Extended emergency time off accrual for employees who accepted through 2020															$ 436,000,000	$ 348,000,000							1,400,000,000
Aircrafts in long-term storage | aircraft													39		39				39	39				39
COVID - 19 | Minimum
Unusual or Infrequent Item, or Both [Line Items]
Extended emergency time off, period										6 months
COVID - 19 | Minimum | Option to Convert to Voluntary Separation Program 2020
Unusual or Infrequent Item, or Both [Line Items]
Extended emergency time off, period										12 months
Years of service										10 years
COVID - 19 | Maximum
Unusual or Infrequent Item, or Both [Line Items]
Extended emergency time off, period										18 months
COVID - 19 | Maximum | Option to Convert to Voluntary Separation Program 2020
Unusual or Infrequent Item, or Both [Line Items]
Extended emergency time off, period										18 months
COVID - 19 | Maximum | Pilots
Unusual or Infrequent Item, or Both [Line Items]
Extended emergency time off, period										5 years
United States Department of Treasury | Subsequent Event | Forecast
Unusual or Infrequent Item, or Both [Line Items]
Payroll support and voluntary Employee programs, net														$ 763,000,000
United States Department of Treasury | COVID - 19
Unusual or Infrequent Item, or Both [Line Items]
Total Payroll Support Proceeds																								$ 7,193,000,000
Promissory Note																								2,068,000,000
Warrants ($)																								$ 85,000,000
Warrants (shares) | shares													4,798,000		4,798,000				4,798,000	4,798,000				4,798,000
United States Department of Treasury | PSP1 Payroll Support Program
Unusual or Infrequent Item, or Both [Line Items]
Total Payroll Support Proceeds							$ 94,000,000	$ 326,000,000	$ 652,000,000		$ 652,000,000	$ 1,630,000,000						$ 3,354,000,000				3,400,000,000
Warrants and rights outstanding																						$ 40,000,000	$ 40,000,000
Class of warrants or rights outstanding (in shares) | shares																						2,700,000	2,700,000
Promissory Note							28,000,000	97,000,000	196,000,000		196,000,000	459,000,000						976,000,000
Warrants ($)							$ 2,000,000	$ 3,000,000	$ 9,000,000		$ 8,000,000	$ 18,000,000						$ 40,000,000
Warrants (shares) | shares							78,000	268,000	536,000		536,000	1,258,000					536,000	78,000			536,000	2,676,000	2,676,000
United States Department of Treasury | PSP1 Payroll Support Program | Notes Payable, Other Payables
Unusual or Infrequent Item, or Both [Line Items]
Principal amount																						$ 976,000,000	$ 976,000,000
United States Department of Treasury | PSP2 Payroll Support Program
Unusual or Infrequent Item, or Both [Line Items]
Total Payroll Support Proceeds				$ 259,000,000	$ 864,000,000	$ 864,000,000													$ 1,987,000,000	$ 2,000,000,000.0
Warrants and rights outstanding													$ 27,000,000		$ 27,000,000				$ 27,000,000	$ 27,000,000				$ 27,000,000
Class of warrants or rights outstanding (in shares) | shares													1,200,000		1,200,000				1,200,000	1,200,000				1,200,000
Promissory Note				78,000,000	259,000,000	229,000,000													$ 566,000,000
Warrants ($)				$ 4,000,000	$ 14,000,000	$ 9,000,000													$ 27,000,000
Warrants (shares) | shares				168,000	560,000	495,000							1,223,000		1,223,000				1,223,000	1,223,000				1,223,000
Strike price (in USD per share) | $ / shares													$ 46.28		$ 46.28				$ 46.28	$ 46.28				$ 46.28
Expiration period (in years)																				5 years
United States Department of Treasury | PSP2 Payroll Support Program | Notes Payable, Other Payables
Unusual or Infrequent Item, or Both [Line Items]
Principal amount													$ 566,000,000		$ 566,000,000				$ 566,000,000	$ 566,000,000				$ 566,000,000
Stated interest rate													1.00%		1.00%				1.00%	1.00%				1.00%
United States Department of Treasury | PSP2 Payroll Support Program | Notes Payable, Other Payables | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | Forecast
Unusual or Infrequent Item, or Both [Line Items]
Variable rate (as a percent)		2.00%
United States Department of Treasury | PSP3 Payroll Support Program
Unusual or Infrequent Item, or Both [Line Items]
Total Payroll Support Proceeds			$ 926,000,000	$ 926,000,000									$ 1,852,000,000		$ 1,900,000,000
Warrants and rights outstanding													$ 18,000,000		$ 18,000,000				$ 18,000,000	$ 18,000,000				$ 18,000,000
Class of warrants or rights outstanding (in shares) | shares													899,000		899,000				899,000	899,000				899,000
Promissory Note			278,000,000	248,000,000									$ 526,000,000
Warrants ($)			$ 9,000,000	$ 9,000,000									$ 18,000,000
Warrants (shares) | shares			475,000	424,000									899,000		899,000				899,000	899,000				899,000
Strike price (in USD per share) | $ / shares													$ 58.51		$ 58.51				$ 58.51	$ 58.51				$ 58.51
Expiration period (in years)																				5 years
United States Department of Treasury | PSP3 Payroll Support Program | Notes Payable, Other Payables
Unusual or Infrequent Item, or Both [Line Items]
Principal amount													$ 526,000,000		$ 526,000,000				$ 526,000,000	$ 526,000,000				$ 526,000,000
Stated interest rate													1.00%		1.00%				1.00%	1.00%				1.00%
United States Department of Treasury | PSP3 Payroll Support Program | Notes Payable, Other Payables | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | Forecast
Unusual or Infrequent Item, or Both [Line Items]
Variable rate (as a percent)	2.00%