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Fair Value Measurements - Schedule of Fair Value Measurements on a Recurring Basis (Details) - Recurring - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Fair Value, Net Asset (Liability)    
Cash equivalents $ 169 $ 2,279
Restricted cash equivalents and investments 39 61
Fuel hedge contracts (139) (324)
Interest rate contract (4) 6
Foreign currency exchange contracts (22) 27
U.S. government and agency securities    
Fair Value, Net Asset (Liability)    
Investments 152 112
Asset- and mortgage-backed securities    
Fair Value, Net Asset (Liability)    
Investments 188 68
Corporate obligations    
Fair Value, Net Asset (Liability)    
Investments 519 295
Other fixed income securities    
Fair Value, Net Asset (Liability)    
Investments 101 12
Long-term investments    
Fair Value, Net Asset (Liability)    
Investments 399 139
Level 1    
Fair Value, Net Asset (Liability)    
Cash equivalents 169 2,279
Restricted cash equivalents and investments 39 61
Fuel hedge contracts (31) (26)
Interest rate contract 0 0
Foreign currency exchange contracts 0 0
Level 1 | U.S. government and agency securities    
Fair Value, Net Asset (Liability)    
Investments 142 86
Level 1 | Asset- and mortgage-backed securities    
Fair Value, Net Asset (Liability)    
Investments 0 0
Level 1 | Corporate obligations    
Fair Value, Net Asset (Liability)    
Investments 0 0
Level 1 | Other fixed income securities    
Fair Value, Net Asset (Liability)    
Investments 0 0
Level 1 | Long-term investments    
Fair Value, Net Asset (Liability)    
Investments 371 115
Level 2    
Fair Value, Net Asset (Liability)    
Cash equivalents 0 0
Restricted cash equivalents and investments 0 0
Fuel hedge contracts (108) (298)
Interest rate contract (4) 6
Foreign currency exchange contracts (22) 27
Level 2 | U.S. government and agency securities    
Fair Value, Net Asset (Liability)    
Investments 10 26
Level 2 | Asset- and mortgage-backed securities    
Fair Value, Net Asset (Liability)    
Investments 188 68
Level 2 | Corporate obligations    
Fair Value, Net Asset (Liability)    
Investments 519 295
Level 2 | Other fixed income securities    
Fair Value, Net Asset (Liability)    
Investments 101 12
Level 2 | Long-term investments    
Fair Value, Net Asset (Liability)    
Investments $ 28 $ 24