Financing Activities - Narrative (Details)		3 Months Ended
	Dec. 05, 2022 USD ($) aircraft	Mar. 31, 2023 USD ($) aircraft	Mar. 31, 2022 USD ($)	Dec. 31, 2022 USD ($) aircraft	May 01, 2020 USD ($) $ / shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Capital expenditures		$ 1,046,000,000	$ 510,000,000
Payments of long-term debt and finance lease obligations		$ 59,000,000	$ 93,000,000
Boeing 737-700 | Air Transportation Equipment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aircrafts to be purchased | aircraft	39	8
Aircrafts to be purchased | aircraft		39		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		28
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		$ 1,600,000,000	$ 2,300,000,000
Stated interest rate					1.25%
Initial conversion price (in USD per share) | $ / shares					$ 38.48
Amortization period (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]
Stated interest rate		1.25%
Amounts outstanding		$ 1,611,000,000
Notes Due 2027
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effective interest rate		1.90%
Line of Credit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Borrowing capacity		$ 1,000,000,000
Amounts outstanding		$ 0