Fair Value Measurements - Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Long-term investments	$ 1,099	$ 1,090
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	586	1,222
Restricted cash equivalents	847	1,183
Long-term investments	1,099	1,090
Recurring | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments		50
Recurring | Asset- and mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments		36
Recurring | Corporate obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments		90
Recurring | Other fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments		27
Recurring | Fuel hedge contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	1	15
Recurring | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	61	1
Recurring | Foreign currency exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	6	(3)
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	586	1,222
Restricted cash equivalents	847	1,183
Long-term investments	881	880
Recurring | Level 1 | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments		45
Recurring | Level 1 | Asset- and mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments		0
Recurring | Level 1 | Corporate obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments		0
Recurring | Level 1 | Other fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments		0
Recurring | Level 1 | Fuel hedge contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	(1)	20
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	218	210
Recurring | Level 2 | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments		5
Recurring | Level 2 | Asset- and mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments		36
Recurring | Level 2 | Corporate obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments		90
Recurring | Level 2 | Other fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term investments		27
Recurring | Level 2 | Fuel hedge contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	2	(5)
Recurring | Level 2 | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	61	1
Recurring | Level 2 | Foreign currency exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Hedge derivatives, net	$ 6	$ (3)