Fair Value Instruments - Carrying and Estimated Fair Value of Debt (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020	May 01, 2020
Unsecured Debt | 2.75% Notes due 2022
Debt Instrument [Line Items]
Stated interest rate	2.75%
Carrying value	$ 300
Unsecured Debt | 4.75% Notes due 2023
Debt Instrument [Line Items]
Stated interest rate	4.75%
Carrying value	$ 1,250
Unsecured Debt | 5.25% Notes due 2025
Debt Instrument [Line Items]
Stated interest rate	5.25%
Carrying value	$ 1,550
Unsecured Debt | 3.00% Notes due 2026
Debt Instrument [Line Items]
Stated interest rate	3.00%
Carrying value	$ 300
Unsecured Debt | 3.45% Notes due 2027
Debt Instrument [Line Items]
Stated interest rate	3.45%
Carrying value	$ 300
Unsecured Debt | 5.125% Notes due 2027
Debt Instrument [Line Items]
Stated interest rate	5.125%
Carrying value	$ 2,000
Unsecured Debt | 7.375% Debentures due 2027
Debt Instrument [Line Items]
Stated interest rate	7.375%
Carrying value	$ 118
Unsecured Debt | 2.625% Notes due 2030
Debt Instrument [Line Items]
Stated interest rate	2.625%
Carrying value	$ 500
Unsecured Debt | 1.000% Payroll Support Program Loan due April 2030
Debt Instrument [Line Items]
Stated interest rate	1.00%
Carrying value	$ 976
Unsecured Debt | 1.000% Payroll Support Program Loan due January 2031
Debt Instrument [Line Items]
Stated interest rate	1.00%
Carrying value	$ 566
Unsecured Debt | 1.000% Payroll Support Program Loan due April 2031
Debt Instrument [Line Items]
Stated interest rate	1.00%
Carrying value	$ 526
Enhanced Equipment Trust Certificate | Pass Through Certificates due 2022 - 6.24%
Debt Instrument [Line Items]
Stated interest rate	6.24%
Carrying value	$ 105
Convertible Debt | 1.25% Convertible Notes due 2025
Debt Instrument [Line Items]
Stated interest rate	1.25%		1.25%
Carrying value	$ 1,982	$ 1,945
Notes Payable to Banks | Term Loan Agreement payable through 2025 - 1.55%
Debt Instrument [Line Items]
Stated interest rate	1.55%
Carrying value	$ 106
Notes Payable to Banks | Term Loan Agreement payable through 2026 - 1.31%
Debt Instrument [Line Items]
Stated interest rate	1.31%
Carrying value	$ 149
Notes Payable to Banks | Term Loan Agreement payable through 2028 - 1.55%
Debt Instrument [Line Items]
Stated interest rate	1.55%
Carrying value	$ 171
Level 2 | Unsecured Debt | 2.75% Notes due 2022
Debt Instrument [Line Items]
Notes payable, Estimated Fair Value	308
Level 2 | Unsecured Debt | 4.75% Notes due 2023
Debt Instrument [Line Items]
Notes payable, Estimated Fair Value	1,342
Level 2 | Unsecured Debt | 5.25% Notes due 2025
Debt Instrument [Line Items]
Notes payable, Estimated Fair Value	1,770
Level 2 | Unsecured Debt | 3.00% Notes due 2026
Debt Instrument [Line Items]
Notes payable, Estimated Fair Value	321
Level 2 | Unsecured Debt | 3.45% Notes due 2027
Debt Instrument [Line Items]
Notes payable, Estimated Fair Value	326
Level 2 | Unsecured Debt | 5.125% Notes due 2027
Debt Instrument [Line Items]
Notes payable, Estimated Fair Value	2,355
Level 2 | Unsecured Debt | 7.375% Debentures due 2027
Debt Instrument [Line Items]
Loans payable, Estimated Fair Value	146
Level 2 | Unsecured Debt | 2.625% Notes due 2030
Debt Instrument [Line Items]
Notes payable, Estimated Fair Value	513
Level 2 | Enhanced Equipment Trust Certificate | Pass Through Certificates due 2022 - 6.24%
Debt Instrument [Line Items]
Notes payable, Estimated Fair Value	108
Level 2 | Convertible Debt | 1.25% Convertible Notes due 2025
Debt Instrument [Line Items]
Notes payable, Estimated Fair Value	3,494
Level 3 | Unsecured Debt | 1.000% Payroll Support Program Loan due April 2030
Debt Instrument [Line Items]
Notes payable, Estimated Fair Value	956
Level 3 | Unsecured Debt | 1.000% Payroll Support Program Loan due January 2031
Debt Instrument [Line Items]
Notes payable, Estimated Fair Value	542
Level 3 | Unsecured Debt | 1.000% Payroll Support Program Loan due April 2031
Debt Instrument [Line Items]
Notes payable, Estimated Fair Value	500
Level 3 | Notes Payable to Banks | Term Loan Agreement payable through 2025 - 1.55%
Debt Instrument [Line Items]
Loans payable, Estimated Fair Value	106
Level 3 | Notes Payable to Banks | Term Loan Agreement payable through 2026 - 1.31%
Debt Instrument [Line Items]
Loans payable, Estimated Fair Value	146
Level 3 | Notes Payable to Banks | Term Loan Agreement payable through 2028 - 1.55%
Debt Instrument [Line Items]
Loans payable, Estimated Fair Value	$ 171