Fair Value Measurements - Schedule of Fair Value Measurements on a Recurring Basis (Details) - Recurring - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Net Asset (Liability)
Cash equivalents	$ 1,648	$ 1,222
Restricted cash equivalents	1,020	1,183
Long-term investments	1,185	1,084
U.S. government and agency securities
Fair Value, Net Asset (Liability)
Short-term investments		50
Asset- and mortgage-backed securities
Fair Value, Net Asset (Liability)
Short-term investments		36
Corporate obligations
Fair Value, Net Asset (Liability)
Short-term investments		90
Other fixed income securities
Fair Value, Net Asset (Liability)
Short-term investments		27
Fuel hedge contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	(4)	15
Interest rate contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	62	1
Foreign currency exchange contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	3	(3)
Level 1
Fair Value, Net Asset (Liability)
Cash equivalents	1,648	1,222
Restricted cash equivalents	1,020	1,183
Long-term investments	980	880
Level 1 | U.S. government and agency securities
Fair Value, Net Asset (Liability)
Short-term investments		45
Level 1 | Asset- and mortgage-backed securities
Fair Value, Net Asset (Liability)
Short-term investments		0
Level 1 | Corporate obligations
Fair Value, Net Asset (Liability)
Short-term investments		0
Level 1 | Other fixed income securities
Fair Value, Net Asset (Liability)
Short-term investments		0
Level 1 | Fuel hedge contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	(18)	20
Level 1 | Interest rate contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	0	0
Level 1 | Foreign currency exchange contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	0	0
Level 2
Fair Value, Net Asset (Liability)
Cash equivalents	0	0
Restricted cash equivalents	0	0
Long-term investments	205	204
Level 2 | U.S. government and agency securities
Fair Value, Net Asset (Liability)
Short-term investments		5
Level 2 | Asset- and mortgage-backed securities
Fair Value, Net Asset (Liability)
Short-term investments		36
Level 2 | Corporate obligations
Fair Value, Net Asset (Liability)
Short-term investments		90
Level 2 | Other fixed income securities
Fair Value, Net Asset (Liability)
Short-term investments		27
Level 2 | Fuel hedge contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	14	(5)
Level 2 | Interest rate contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	62	1
Level 2 | Foreign currency exchange contracts
Fair Value, Net Asset (Liability)
Hedge derivatives, net	$ 3	$ (3)