Fair Value Measurements - Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Long-term investments	$ 1,417	$ 1,099
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	5,755	586
Restricted cash equivalents	1,747	847
Long-term investments	1,417	1,099
Recurring | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments	5,789
Recurring | Fuel hedge contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	(9)	1
Recurring | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	23	61
Recurring | Foreign currency exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	(13)	6
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	5,755	586
Restricted cash equivalents	1,747	847
Long-term investments	948	881
Recurring | Level 1 | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments	3,919
Recurring | Level 1 | Fuel hedge contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	0	(1)
Recurring | Level 1 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	0	0
Recurring | Level 1 | Foreign currency exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	0	0
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	0	0
Restricted cash equivalents	0	0
Long-term investments	38	33
Recurring | Level 2 | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments	1,870
Recurring | Level 2 | Fuel hedge contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	(9)	2
Recurring | Level 2 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	23	61
Recurring | Level 2 | Foreign currency exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	(13)	6
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	0	0
Restricted cash equivalents	0	0
Long-term investments	431	185
Recurring | Level 3 | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments	0
Recurring | Level 3 | Fuel hedge contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	0	0
Recurring | Level 3 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	0	0
Recurring | Level 3 | Foreign currency exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	$ 0	$ 0