Financing Activities - Narrative (Details)			1 Months Ended	3 Months Ended				9 Months Ended
	Sep. 30, 2024 USD ($) aircraft	May 01, 2020 USD ($) $ / shares	Jun. 30, 2024 USD ($)	Sep. 30, 2024 USD ($) aircraft	Jun. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Mar. 31, 2023 USD ($) aircraft	Sep. 30, 2024 USD ($) aircraft	Sep. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($) aircraft	Dec. 05, 2022 aircraft	Dec. 31, 2021 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Value of warrant repurchase and termination			$ 6,000,000	$ 0	$ 6,000,000	$ 0		$ 6,000,000	$ 0
Capital expenditures				517,000,000		842,000,000		1,594,000,000	2,812,000,000
Payments of long-term debt and finance lease obligations				$ 11,000,000		11,000,000		$ 27,000,000	78,000,000
Boeing 737-700 | Air Transportation Equipment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of aircrafts to be purchased | aircraft												39
Number of aircrafts purchased | aircraft							8				31
Capital expenditures							$ 88,000,000
Payments of long-term debt and finance lease obligations							$ 50,000,000
Total lease obligations						38,000,000			38,000,000
Number of leased aircrafts | aircraft	19			19				19
Gain (loss) on sale of leased aircraft								$ 0
Convertible Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal amount	$ 1,600,000,000	$ 2,300,000,000		$ 1,600,000,000				$ 1,600,000,000		$ 1,600,000,000
Conversion ratio	0.0271951	0.0259909
Initial conversion price (in USD per share) | $ / shares		$ 38.48
Principal amount of debt repurchased											$ 689,000,000		$ 689,000,000
Term (in years)								5 years
Convertible Debt | 1.25% Convertible Notes due 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amounts outstanding	$ 1,611,000,000			$ 1,611,000,000				$ 1,611,000,000
Convertible Debt | 1.25% Convertible Notes due 2025 | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate	1.25%	1.25%		1.25%				1.25%
Line of Credit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Borrowing capacity	$ 1,000,000,000			$ 1,000,000,000				$ 1,000,000,000
Amounts outstanding	$ 0			$ 0		$ 0		$ 0	$ 0
Notes Due 2027
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effective interest rate	1.90%			1.90%				1.90%