Fair Value Measurements - Schedule of Fair Value Measurement (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	$ 2,279	$ 1,543
Restricted cash equivalents and investments	61	49
Fuel hedge contracts	(324)	(672)
Interest rate contract	6	(3)
Foreign currency exchange contracts	27	94
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	2,279	1,543
Restricted cash equivalents and investments	61	49
Fuel hedge contracts	(26)	65
Interest rate contract	0	0
Foreign currency exchange contracts	0	0
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	(298)	(737)
Interest rate contract	6	(3)
Foreign currency exchange contracts	27	94
U.S. government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	112	151
U.S. government and agency securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	86	74
U.S. government and agency securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	26	77
Asset- and mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	68	380
Asset- and mortgage-backed securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	0	0
Asset- and mortgage-backed securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	68	380
Corporate obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	295	896
Corporate obligations | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	0	0
Corporate obligations | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	295	896
Other fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	12	38
Other fixed income securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	0	0
Other fixed income securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	12	38
Long-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	139	155
Long-term investments | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	115	130
Long-term investments | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investments	$ 24	$ 25