Fair Value Instruments - Carrying and Estimated Fair Value of Debt (Details) - USD ($) $ in Millions	Jun. 30, 2020	Jun. 08, 2020	May 04, 2020	May 01, 2020
2.65% Notes due 2020 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 502
Stated interest rate	2.65%
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	$ 168
Stated interest rate	6.24%
4.750 Unsecured Notes due 2023
Debt Instrument [Line Items]
Stated interest rate		4.75%	4.75%
4.750 Unsecured Notes due 2023 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 1,250
Stated interest rate	4.75%
1.250 Convertible Senior Notes due 2025
Debt Instrument [Line Items]
Stated interest rate				1.25%
1.250 Convertible Senior Notes due 2025 | Unsecured Debt
Debt Instrument [Line Items]
Stated interest rate	1.25%
1.250 Convertible Senior Notes due 2025 | Convertible Debt [Member]
Debt Instrument [Line Items]
Carrying amount of debt	$ 1,909
Stated interest rate				1.25%
5.250 Unsecured Notes due 2025
Debt Instrument [Line Items]
Stated interest rate			5.25%
5.250 Unsecured Notes due 2025 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 1,250
Stated interest rate	5.25%
3.00% Notes due 2026 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 300
Stated interest rate	3.00%
Term Loan Agreement due 2026 - 1.99% | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 169
Stated interest rate	1.99%
3.45% Notes due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Stated interest rate	3.45%
5.125 Notes due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 1,300
Stated interest rate	5.125%
7.375% Debentures due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 121
Stated interest rate	7.375%
1.71 Term Loan Agreement Payable through 2028 | Notes Payable to Banks
Debt Instrument [Line Items]
Carrying amount of debt	$ 196
2.625% Notes due 2030
Debt Instrument [Line Items]
Stated interest rate	2.625%
2.625% Notes due 2030 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 500
Stated interest rate	2.625%
1.0 Payroll Support Program Loan due 2030 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 850
Stated interest rate	1.00%
3.45 Unsecured Senior Notes due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt	$ 300
Level 2 | 2.65% Notes due 2020 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	503
Level 2 | 2.75% Notes due 2022 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	304
Level 2 | Pass Through Certificates due 2022 - 6.24% | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Notes Payable, Fair Value	172
Level 2 | 4.750 Unsecured Notes due 2023
Debt Instrument [Line Items]
Notes Payable, Fair Value	1,289
Level 2 | 1.250 Convertible Senior Notes due 2025 | Convertible Debt [Member]
Debt Instrument [Line Items]
Notes Payable, Fair Value	2,762
Level 2 | 5.250 Unsecured Notes due 2025
Debt Instrument [Line Items]
Notes Payable, Fair Value	1,317
Level 2 | 3.00% Notes due 2026 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	296
Level 2 | 5.125 Notes due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	1,352
Level 2 | 7.375% Debentures due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Loans Payable, Fair Value	134
Level 2 | 2.625% Notes due 2030 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	455
Level 2 | 3.45 Unsecured Senior Notes due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	294
Level 3 | Term Loan Agreement due 2026 - 1.99% | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	159
Level 3 | 1.71 Term Loan Agreement Payable through 2028 | Notes Payable to Banks
Debt Instrument [Line Items]
Loans Payable, Fair Value	176
Level 3 | 1.0 Payroll Support Program Loan due 2030 | Unsecured Debt
Debt Instrument [Line Items]
Notes Payable, Fair Value	$ 629
Notes Payable to Banks | 1.71 Term Loan Agreement Payable through 2028
Debt Instrument [Line Items]
Stated interest rate	1.71%