Fair Value Instruments - Carrying and Estimated Fair Value of Debt (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Jul. 31, 2020	May 04, 2020	May 01, 2020	Feb. 29, 2020	Nov. 30, 2017	Nov. 30, 2016	Feb. 28, 1997
2.75% Notes due 2022
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)								2.75%
2.75% Notes due 2022 | Unsecured Debt
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)	2.75%
Carrying value	$ 0	$ 300
Pass Through Certificates due 2022 - 6.24% | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)	6.24%
Carrying value	$ 0	71
4.75% Notes due 2023
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)					4.75%
4.75% Notes due 2023 | Unsecured Debt
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)	4.75%
Carrying value	$ 0	1,250
1.25% Convertible Notes due 2025 | Unsecured Debt
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)	1.25%
1.25% Convertible Notes due 2025 | Convertible Notes
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)						1.25%
Carrying value	$ 1,611	1,842
5.25% Notes due 2025 | Unsecured Debt
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)	5.25%		5.25%
Carrying value	$ 1,302	1,550		$ 300
3.00% Notes due 2026
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)									3.00%
3.00% Notes due 2026 | Unsecured Debt
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)	3.00%
Carrying value	$ 300	300
3.45% Notes due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)	3.45%
Carrying value	$ 300	300
5.125% Notes due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)	5.125%		5.125%
Carrying value	$ 1,727	2,000		$ 700
7.375% Debentures due 2027
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)										7.375%
7.375% Debentures due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)	7.375%
Carrying value	$ 114	116
2.625% Notes due 2030 | Unsecured Debt
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)							2.625%
Carrying value	$ 500	500
1.000% Payroll Support Program Loan due April 2030 (See Note 2) | Unsecured Debt
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)	1.00%
Carrying value	$ 976	976
1.000% Payroll Support Program Loan due January 2031 (See Note 2) | Unsecured Debt
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)	1.00%
Carrying value	$ 566	566
1.000% Payroll Support Program Loan due April 2031 (See Note 2) | Unsecured Debt
Debt Instrument [Line Items]
Interest rate stated in the debt agreement (as a percent)	1.00%
Carrying value	$ 526	$ 526
Significant other observable inputs (Level 2) | 1.25% Convertible Notes due 2025 | Convertible Notes
Debt Instrument [Line Items]
Estimated fair value	1,917
Significant other observable inputs (Level 2) | 5.25% Notes due 2025
Debt Instrument [Line Items]
Estimated fair value	1,308
Significant other observable inputs (Level 2) | 3.00% Notes due 2026 | Unsecured Debt
Debt Instrument [Line Items]
Estimated fair value	276
Significant other observable inputs (Level 2) | 3.45% Notes due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Estimated fair value	276
Significant other observable inputs (Level 2) | 5.125% Notes due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Estimated fair value	1,713
Significant other observable inputs (Level 2) | 7.375% Debentures due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Estimated fair value	120
Significant other observable inputs (Level 2) | 2.625% Notes due 2030 | Unsecured Debt
Debt Instrument [Line Items]
Estimated fair value	415
Significant other observable inputs (Level 2) | 1.000% Payroll Support Program Loan due January 2031 (See Note 2) | Unsecured Debt
Debt Instrument [Line Items]
Estimated fair value	475
Level 3 | 1.000% Payroll Support Program Loan due April 2030 (See Note 2) | Unsecured Debt
Debt Instrument [Line Items]
Estimated fair value	850
Level 3 | 1.000% Payroll Support Program Loan due April 2031 (See Note 2) | Unsecured Debt
Debt Instrument [Line Items]
Estimated fair value	$ 436