Fair Value Measurements - Schedule of Fair Value Measurements on a Recurring Basis (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 1,618	$ 1,612
Restricted cash equivalents and investments	45	37
Fuel hedge contracts	(1,428)	(1,848)
Interest rate contract	(1)	(7)
Foreign currency exchange contracts	96	73
Short-Term Investments | U.S. government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	135	59
Short-Term Investments | Asset- and mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	395	392
Short-Term Investments | Corporate obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	894	749
Short-Term Investments | Other fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	23	17
Long-Term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	99	118
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	1,618	1,612
Restricted cash equivalents and investments	45	37
Fuel hedge contracts	(68)	(167)
Interest rate contract	0	0
Foreign currency exchange contracts	0	0
Level 1 | Short-Term Investments | U.S. government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	63	0
Level 1 | Short-Term Investments | Asset- and mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Level 1 | Short-Term Investments | Corporate obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Level 1 | Short-Term Investments | Other fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Level 1 | Long-Term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	73	90
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Restricted cash equivalents and investments	0	0
Fuel hedge contracts	(1,360)	(1,681)
Interest rate contract	(1)	(7)
Foreign currency exchange contracts	96	73
Level 2 | Short-Term Investments | U.S. government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	72	59
Level 2 | Short-Term Investments | Asset- and mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	395	392
Level 2 | Short-Term Investments | Corporate obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	894	749
Level 2 | Short-Term Investments | Other fixed income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	23	17
Level 2 | Long-Term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	$ 26	$ 28