Financing Activities - Narrative (Details)			3 Months Ended
	Mar. 31, 2024 USD ($) aircraft	May 01, 2020 USD ($) $ / shares	Mar. 31, 2024 USD ($) aircraft	Mar. 31, 2023 USD ($) aircraft	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($) aircraft	Dec. 05, 2022 aircraft
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Capital expenditures			$ 583,000,000	$ 1,046,000,000
Payments of long-term debt and finance lease obligations			$ 8,000,000	$ 59,000,000
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
Number of leased aircrafts | aircraft	24		24
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
Conversion ratio	0.0268605	0.0259909
Initial conversion price (in USD per share) | $ / shares		$ 38.48
Principal amount of debt repurchased						$ 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
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
Amounts outstanding	$ 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%