Fair Value Instruments - Carrying and Estimated Fair Value of Debt (Details) $ in Millions	Sep. 30, 2016 USD ($)
5.25% Convertible Senior Notes due 2016 | Convertible Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 110
Stated interest rate	5.25%
5.75% Notes due 2016 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 302
Stated interest rate	5.75%
5.125% Notes due 2017 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 303
Stated interest rate	5.125%
French Credit Agreements Due 2018 - 1.96% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 19
Stated interest rate	1.96%
Fixed-rate 737 Aircraft Notes payable through 2018 - 7.03% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 9
Stated interest rate	7.03%
2.75% Notes Due 2019 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 307
Stated interest rate	2.75%
Term Loan Agreement due 2019 - 6.315% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 115
Stated interest rate	6.315%
Term Loan Agreement due 2019 - 4.84% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 28
Stated interest rate	4.84%
2.65% Notes due 2020 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 507
Stated interest rate	2.65%
Term Loan Agreement due 2020 - 5.223% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 296
Stated interest rate	5.223%
Pass Through Certificates due 2022 - 6.24% | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Carrying amount of debt	$ 324
Stated interest rate	6.24%
Floating-rate 737 Aircraft Notes payable through 2020 | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 217
Term Loan Agreement Due 2021 - 7.94% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 20
Stated interest rate	7.94%
7.375% Debentures due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 130
Stated interest rate	7.375%
Level 2 | 5.25% Convertible Senior Notes due 2016 | Convertible Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	$ 301
Level 2 | 5.75% Notes due 2016 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	304
Level 2 | 5.125% Notes due 2017 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	308
Level 2 | 2.75% Notes Due 2019 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	316
Level 2 | 2.65% Notes due 2020 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	521
Level 2 | Pass Through Certificates due 2022 - 6.24% | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Notes Payable, Fair Value	362
Level 2 | 7.375% Debentures due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Loans Payable, Fair Value	161
Level 3 | French Credit Agreements Due 2018 - 1.96% | Notes Payable to Banks
Debt Instrument [Line Items]
Notes Payable, Fair Value	19
Level 3 | Fixed-rate 737 Aircraft Notes payable through 2018 - 7.03% | Notes Payable to Banks
Debt Instrument [Line Items]
Notes Payable, Fair Value	10
Level 3 | Term Loan Agreement due 2019 - 6.315% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	117
Level 3 | Term Loan Agreement due 2019 - 4.84% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	29
Level 3 | Term Loan Agreement due 2020 - 5.223% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	291
Level 3 | Floating-rate 737 Aircraft Notes payable through 2020 | Notes Payable to Banks
Debt Instrument [Line Items]
Notes Payable, Fair Value	213
Level 3 | Term Loan Agreement Due 2021 - 7.94% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	$ 22