Fair Value Instruments - Carrying and Estimated Fair Value of Debt (Details) $ in Millions	Sep. 30, 2018 USD ($)
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	$ 34
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	$ 10
Stated interest rate	4.84%
2.65% Notes due 2020 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 485
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	$ 199
Stated interest rate	5.223%
737 Aircraft Notes payable through 2020 | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 100
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	$ 250
Stated interest rate	6.24%
Term Loan Agreement due 2026 - 3.62% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 206
Stated interest rate	3.62%
3.00% Notes due 2026 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 300
Stated interest rate	3.00%
3.45% Notes due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 300
Stated interest rate	3.45%
7.375% Debentures due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 125
Stated interest rate	7.375%
Level 2 | 2.75% Notes Due 2019 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	$ 297
Level 2 | 2.65% Notes due 2020 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	480
Level 2 | 2.75% Notes due 2022 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	290
Level 2 | Pass Through Certificates due 2022 - 6.24% | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Notes Payable, Fair Value	264
Level 2 | 3.00% Notes due 2026 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	276
Level 2 | 3.45% Notes due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	283
Level 2 | 7.375% Debentures due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Loans Payable, Fair Value	145
Level 3 | Term Loan Agreement payable through 2019 - 6.315% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	34
Level 3 | Term Loan Agreement payable through 2019 - 4.84% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	10
Level 3 | Term Loan Agreement payable through 2020 - 5.223% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	200
Level 3 | 737 Aircraft Notes payable through 2020 | Notes Payable to Banks
Debt Instrument [Line Items]
Notes Payable, Fair Value	99
Level 3 | Term Loan Agreement due 2026 - 3.62% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	$ 206