Fair Value Instruments - Carrying and Estimated Fair Value of Debt (Details) - USD ($) $ in Millions	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
Convertible Notes
Debt Instrument [Line Items]
Carrying value	$ 2,097	$ 2,300
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	$ 300	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	$ 71	137
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	$ 1,250	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	$ 1,842	$ 1,945
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	$ 1,550	$ 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	$ 2,000	$ 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	$ 116	119
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	0
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	$ 0
Significant other observable inputs (Level 2) | 2.75% Notes due 2022 | Unsecured Debt
Debt Instrument [Line Items]
Estimated fair value	305
Significant other observable inputs (Level 2) | Pass Through Certificates due 2022 - 6.24% | Enhanced Equipment Trust Certificate
Debt Instrument [Line Items]
Estimated fair value	73
Significant other observable inputs (Level 2) | 4.75% Notes due 2023
Debt Instrument [Line Items]
Estimated fair value	1,311
Significant other observable inputs (Level 2) | 1.25% Convertible Notes due 2025 | Convertible Notes
Debt Instrument [Line Items]
Estimated fair value	2,802
Significant other observable inputs (Level 2) | 5.25% Notes due 2025
Debt Instrument [Line Items]
Estimated fair value	1,724
Significant other observable inputs (Level 2) | 3.00% Notes due 2026 | Unsecured Debt
Debt Instrument [Line Items]
Estimated fair value	314
Significant other observable inputs (Level 2) | 3.45% Notes due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Estimated fair value	321
Significant other observable inputs (Level 2) | 5.125% Notes due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Estimated fair value	2,294
Significant other observable inputs (Level 2) | 7.375% Debentures due 2027 | Unsecured Debt
Debt Instrument [Line Items]
Estimated fair value	140
Significant other observable inputs (Level 2) | 2.625% Notes due 2030 | Unsecured Debt
Debt Instrument [Line Items]
Estimated fair value	499
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	548
Level 3 | 1.000% Payroll Support Program Loan due April 2030 (See Note 2) | Unsecured Debt
Debt Instrument [Line Items]
Estimated fair value	958
Level 3 | 1.000% Payroll Support Program Loan due April 2031 (See Note 2) | Unsecured Debt
Debt Instrument [Line Items]
Estimated fair value	$ 507