Fair Value Measurements - Measured on Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Fuel derivatives:
Option contracts	$ 280	$ 223
Fair Value, Measurements, Recurring
Assets
Treasury bills	1,934	1,983
Fuel derivatives:
Equity Securities	286	280
Total assets	11,078	11,977
Fair Value, Measurements, Recurring | Option contracts
Fuel derivatives:
Option contracts	280	223
Fair Value, Measurements, Recurring | Option contracts
Liabilities
Option contracts	(23)
Fair Value, Measurements, Recurring | Certificates of deposit
Assets
Certificates of deposit	211	203
Fair Value, Measurements, Recurring | Cash equivalents
Assets
Cash equivalents	8,119	9,032
Fair Value, Measurements, Recurring | Commercial paper
Assets
Cash equivalents	135	135
Fair Value, Measurements, Recurring | Certificates of deposit
Assets
Cash equivalents	13	21
Fair Value, Measurements, Recurring | Time deposits
Assets
Cash equivalents	100	100
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Treasury bills	1,934	1,983
Fuel derivatives:
Equity Securities	286	280
Total assets	10,339	11,295
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1) | Option contracts
Fuel derivatives:
Option contracts	0	0
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1) | Option contracts
Liabilities
Option contracts	0
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1) | Certificates of deposit
Assets
Certificates of deposit	0	0
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1) | Cash equivalents
Assets
Cash equivalents	8,119	9,032
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1) | Commercial paper
Assets
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1) | Certificates of deposit
Assets
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1) | Time deposits
Assets
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Treasury bills	0	0
Fuel derivatives:
Equity Securities	0	0
Total assets	459	459
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Option contracts
Fuel derivatives:
Option contracts	0	0
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Option contracts
Liabilities
Option contracts	0
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Certificates of deposit
Assets
Certificates of deposit	211	203
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Cash equivalents
Assets
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Commercial paper
Assets
Cash equivalents	135	135
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Certificates of deposit
Assets
Cash equivalents	13	21
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Time deposits
Assets
Cash equivalents	100	100
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Treasury bills	0	0
Fuel derivatives:
Equity Securities	0	0
Total assets	280	223
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Option contracts
Fuel derivatives:
Option contracts	280	223
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Option contracts
Liabilities
Option contracts	(23)
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Certificates of deposit
Assets
Certificates of deposit	0	0
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Cash equivalents
Assets
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Commercial paper
Assets
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Certificates of deposit
Assets
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Time deposits
Assets
Cash equivalents	$ 0	$ 0