Fair Value Instruments - Carrying and Estimated Fair Value of Debt (Details) $ in Millions	Mar. 31, 2016 USD ($)
5.75% Notes due 2016 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 305
Stated interest rate	5.75%
5.25% Convertible Senior Notes due 2016 | Convertible Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 111
Stated interest rate	5.25%
5.125% Notes due 2017 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 307
Stated interest rate	5.125%
French Credit Agreements Due 2018 - 1.53% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 25
Stated interest rate	1.53%
Fixed-rate 737 Aircraft Notes payable through 2018 - 7.02% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 15
Stated interest rate	7.02%
2.75% Notes Due 2019 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 303
Stated interest rate	2.75%
Term Loan Agreement due 2019 - 6.315% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 134
Stated interest rate	6.315%
Term Loan Agreement due 2019 - 4.84% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 32
Stated interest rate	4.84%
2.65% Notes due 2020 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 514
Term Loan Agreement due 2020 - 5.223% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 318
Stated interest rate	5.223%
Floating-rate 737 Aircraft Notes payable through 2020 | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 245
Pass Through Certificates due 2022 - 6.24% | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Carrying amount of debt	$ 331
Stated interest rate	6.24%
7.375% Debentures due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 131
Stated interest rate	7.375%
Level 2 | 5.75% Notes due 2016 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	$ 314
Level 2 | 5.25% Convertible Senior Notes due 2016 | Convertible Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	335
Level 2 | 5.125% Notes due 2017 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	317
Level 2 | 2.75% Notes Due 2019 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	312
Level 2 | 2.65% Notes due 2020 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	526
Level 2 | Pass Through Certificates due 2022 - 6.24% | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Notes Payable, Fair Value	374
Level 2 | 7.375% Debentures due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Loans Payable, Fair Value	159
Level 3 | French Credit Agreements Due 2018 - 1.53% | Notes Payable to Banks
Debt Instrument [Line Items]
Notes Payable, Fair Value	25
Level 3 | Fixed-rate 737 Aircraft Notes payable through 2018 - 7.02% | Notes Payable to Banks
Debt Instrument [Line Items]
Notes Payable, Fair Value	15
Level 3 | Term Loan Agreement due 2019 - 6.315% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	138
Level 3 | Term Loan Agreement due 2019 - 4.84% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	34
Level 3 | Term Loan Agreement due 2020 - 5.223% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	313
Level 3 | Floating-rate 737 Aircraft Notes payable through 2020 | Notes Payable to Banks
Debt Instrument [Line Items]
Notes Payable, Fair Value	$ 242