Fair Value Measurements - Schedule of Fair Value Measurement (Details) - Recurring - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	$ 1,357	$ 2,279
Restricted cash equivalents and investments	38	61
Fuel hedge contracts	(66)	(324)
Foreign currency exchange contracts	(17)	27
U.S. government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	93	112
Asset- and mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	173	68
Corporate obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	467	295
Other fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	92	12
Long-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	513	139
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	1,357	2,279
Restricted cash equivalents and investments	38	61
Fuel hedge contracts	(43)	(26)
Foreign currency exchange contracts	0	0
Level 1 | U.S. government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	84	86
Level 1 | Asset- and mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	0	0
Level 1 | Corporate obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	0	0
Level 1 | Other fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	0	0
Level 1 | Long-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	485	115
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	0	0
Restricted cash equivalents and investments	0	0
Fuel hedge contracts	(23)	(298)
Foreign currency exchange contracts	(17)	27
Level 2 | U.S. government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	9	26
Level 2 | Asset- and mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	173	68
Level 2 | Corporate obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	467	295
Level 2 | Other fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	92	12
Level 2 | Long-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	$ 28	$ 24