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Fair Value Instruments - Carrying and Estimated Fair Value of Debt (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020
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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 | $ 105 |
Stated interest rate | 6.24% |
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 | 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] |  |
Net carrying amount | $ 1,963 | $ 1,945
5.250 Unsecured Notes due 2025 | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Carrying amount of debt | $ 1,550 |
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 | $ 159 |
Stated interest rate | 1.34% |
5.125 Notes due 2027 | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Carrying amount of debt | $ 2,000 |
Stated interest rate | 5.125% |
7.375% Debentures due 2027 | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Carrying amount of debt | $ 119 |
Stated interest rate | 7.375% |
1.60 Term Loan Agreement Payable through 2028 | Notes Payable to Banks |  |
Debt Instrument [Line Items] |  |
Carrying amount of debt | $ 178 |
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 | $ 976 |
Stated interest rate | 1.00% |
3.45% Notes due 2027 | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Carrying amount of debt | $ 300 |
Stated interest rate | 3.45% |
Term Loan Agreement Due 2025 | Notes Payable to Banks |  |
Debt Instrument [Line Items] |  |
Carrying amount of debt | $ 112 |
Stated interest rate | 1.59% |
Term Loan Agreement Due 2028 | Notes Payable to Banks |  |
Debt Instrument [Line Items] |  |
Stated interest rate | 1.60% |
1.0 Payroll Support Program Loan due 2031 | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Carrying amount of debt | $ 488 |
Stated interest rate | 1.00% |
Level 2 | 2.75% Notes due 2022 | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Notes Payable, Fair Value | $ 309 |
Level 2 | Pass Through Certificates due 2022 - 6.24% | Enhanced Equipment Trust Certificate |  |
Debt Instrument [Line Items] |  |
Notes Payable, Fair Value | 107 |
Level 2 | 4.750 Unsecured Notes due 2023 |  |
Debt Instrument [Line Items] |  |
Notes Payable, Fair Value | 1,350 |
Level 2 | 1.250 Convertible Senior Notes due 2025 | Convertible Debt [Member] |  |
Debt Instrument [Line Items] |  |
Notes Payable, Fair Value | 3,968 |
Level 2 | 5.250 Unsecured Notes due 2025 |  |
Debt Instrument [Line Items] |  |
Notes Payable, Fair Value | 1,765 |
Level 2 | 3.00% Notes due 2026 | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Notes Payable, Fair Value | 317 |
Level 2 | 5.125 Notes due 2027 | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Notes Payable, Fair Value | 2,301 |
Level 2 | 7.375% Debentures due 2027 | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Loans Payable, Fair Value | 145 |
Level 2 | 2.625% Notes due 2030 | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Notes Payable, Fair Value | 492 |
Level 2 | 3.45% Notes due 2027 | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Notes Payable, Fair Value | 318 |
Level 3 | Term Loan Agreement due 2026 - 1.99% | Notes Payable to Banks |  |
Debt Instrument [Line Items] |  |
Loans Payable, Fair Value | 157 |
Level 3 | 1.60 Term Loan Agreement Payable through 2028 | Notes Payable to Banks |  |
Debt Instrument [Line Items] |  |
Loans Payable, Fair Value | 177 |
Level 3 | 1.0 Payroll Support Program Loan due 2030 | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Notes Payable, Fair Value | 941 |
Level 3 | Term Loan Agreement Due 2025 | Notes Payable to Banks |  |
Debt Instrument [Line Items] |  |
Loans Payable, Fair Value | 112 |
Level 3 | 1.0 Payroll Support Program Loan due 2031 | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Notes Payable, Fair Value | $ 460 |

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- Definition Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 + Details
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- Definition Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)) -URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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