Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Net Asset (Liability)
Long-term investments	$ 1,310	$ 1,099
Recurring
Fair Value, Net Asset (Liability)
Cash equivalents	13,554	586
Restricted cash equivalents	1,868	847
Long-term investments	1,310	1,099
Recurring | U.S. Government securities
Fair Value, Net Asset (Liability)
Short-term investments	5,048
Recurring | Fuel hedge contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	(3)	1
Recurring | Interest rate contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	25	61
Recurring | Foreign currency exchange contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	(2)	6
Recurring | Level 1
Fair Value, Net Asset (Liability)
Cash equivalents	13,554	586
Restricted cash equivalents	1,868	847
Long-term investments	846	881
Recurring | Level 1 | U.S. Government securities
Fair Value, Net Asset (Liability)
Short-term investments	3,694
Recurring | Level 1 | Fuel hedge contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	0	(1)
Recurring | Level 1 | Interest rate contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	0	0
Recurring | Level 1 | Foreign currency exchange contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	0	0
Recurring | Level 2
Fair Value, Net Asset (Liability)
Cash equivalents	0	0
Restricted cash equivalents	0	0
Long-term investments	230	218
Recurring | Level 2 | U.S. Government securities
Fair Value, Net Asset (Liability)
Short-term investments	1,354
Recurring | Level 2 | Fuel hedge contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	(3)	2
Recurring | Level 2 | Interest rate contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	25	61
Recurring | Level 2 | Foreign currency exchange contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	(2)	$ 6
Recurring | Level 3
Fair Value, Net Asset (Liability)
Cash equivalents	0
Restricted cash equivalents	0
Long-term investments	234
Recurring | Level 3 | U.S. Government securities
Fair Value, Net Asset (Liability)
Short-term investments	0
Recurring | Level 3 | Fuel hedge contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	0
Recurring | Level 3 | Interest rate contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	0
Recurring | Level 3 | Foreign currency exchange contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	$ 0