Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments	$ 3,386	$ 5,789
Short-term investments expected to mature in one year or less	2,800
Fuel hedge contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Gain (loss) recognized on derivatives	$ (146)	$ 85	$ (41)