Fair Value Instruments - Carrying and Estimated Fair Value of Debt (Details) $ in Millions	Sep. 30, 2017 USD ($)
French Credit Agreements Due 2018 - 2.15% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 7
Stated interest rate	2.15%
Fixed-rate 737 Aircraft Notes payable through 2018 - 7.03% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 3
Stated interest rate	7.03%
2.75% Notes Due 2019 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 301
Stated interest rate	2.75%
Term Loan Agreement payable through 2019 - 6.315% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 76
Stated interest rate	6.315%
Term Loan Agreement payable through 2019 - 4.84% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 19
Stated interest rate	4.84%
2.65% Notes due 2020 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 495
Stated interest rate	2.65%
Term Loan Agreement payable through 2020 - 5.223% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 249
Stated interest rate	5.223%
737 Aircraft Notes payable through 2020 | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 166
Pass Through Certificates due 2022 - 6.24% | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Carrying amount of debt	$ 294
Stated interest rate	6.24%
Term Loan Agreement due 2026 - 2.53% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 215
Stated interest rate	2.53%
3.00% Notes due 2026 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 300
Stated interest rate	3.00%
7.375% Debentures due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 128
Stated interest rate	7.375%
Level 2 | 2.75% Notes Due 2019 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	$ 305
Level 2 | 2.65% Notes due 2020 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	503
Level 2 | Pass Through Certificates due 2022 - 6.24% | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Notes Payable, Fair Value	322
Level 2 | 3.00% Notes due 2026 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	296
Level 2 | 7.375% Debentures due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Loans Payable, Fair Value	157
Level 3 | French Credit Agreements Due 2018 - 2.15% | Notes Payable to Banks
Debt Instrument [Line Items]
Notes Payable, Fair Value	7
Level 3 | Fixed-rate 737 Aircraft Notes payable through 2018 - 7.03% | Notes Payable to Banks
Debt Instrument [Line Items]
Notes Payable, Fair Value	3
Level 3 | Term Loan Agreement payable through 2019 - 6.315% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	77
Level 3 | Term Loan Agreement payable through 2019 - 4.84% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	20
Level 3 | Term Loan Agreement payable through 2020 - 5.223% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	252
Level 3 | 737 Aircraft Notes payable through 2020 | Notes Payable to Banks
Debt Instrument [Line Items]
Notes Payable, Fair Value	165
Level 3 | Term Loan Agreement due 2026 - 2.53% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	$ 215