Fair Value Instruments - Carrying and Estimated Fair Value of Debt (Details) $ in Millions	Mar. 31, 2018 USD ($)
French Credit Agreements Due 2018 - 2.54% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 1
Stated interest rate	2.54%
2.75% Notes Due 2019 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 298
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	$ 55
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	$ 15
Stated interest rate	4.84%
2.65% Notes due 2020 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 486
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	$ 225
Stated interest rate	5.223%
737 Aircraft Notes payable through 2020 | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 144
2.75% Notes due 2022 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 300
Stated interest rate	2.75%
Pass Through Certificates due 2022 - 6.24% | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Carrying amount of debt	$ 273
Stated interest rate	6.24%
Term Loan Agreement due 2026 - 2.67% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 215
Stated interest rate	2.67%
3.00% Notes due 2026 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 300
Stated interest rate	3.00%
Unsecured Senior Notes Due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 300
Stated interest rate	3.45%
Debentures due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 126
Stated interest rate	7.375%
Level 2 | 2.75% Notes Due 2019 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	$ 298
Level 2 | 2.65% Notes due 2020 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	483
Level 2 | 2.75% Notes due 2022 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	295
Level 2 | Pass Through Certificates due 2022 - 6.24% | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Notes Payable, Fair Value	293
Level 2 | 3.00% Notes due 2026 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	284
Level 2 | Unsecured Senior Notes Due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	292
Level 2 | Debentures due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Loans Payable, Fair Value	151
Level 3 | French Credit Agreements Due 2018 - 2.54% | Notes Payable to Banks
Debt Instrument [Line Items]
Notes Payable, Fair Value	1
Level 3 | Term Loan Agreement payable through 2019 - 6.315% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	55
Level 3 | Term Loan Agreement payable through 2019 - 4.84% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	15
Level 3 | Term Loan Agreement payable through 2020 - 5.223% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	225
Level 3 | 737 Aircraft Notes payable through 2020 | Notes Payable to Banks
Debt Instrument [Line Items]
Notes Payable, Fair Value	144
Level 3 | Term Loan Agreement due 2026 - 2.67% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	$ 215