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Loans Payable, Long-Term Debt and Leases - Narrative (Details) - USD ($) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Jun. 30, 2020
Debt Instrument [Line Items] |  |  |  |
Long-term debt, current maturities | $ 2,300,000,000 | $ 1,900,000,000 |  |
Loans payable and current portion of long-term debt | 6,431,000,000 | 3,610,000,000 |  |
Long-term debt | 25,360,000,000 | $ 22,736,000,000 |  |
Face amount of debt |  |  |  | $ 4,500,000,000
Long-term debt, maturities, repayments of principal in 2021 | 2,300,000,000 |  |  |
Long-term debt, maturities, repayments of principal in 2022 | 2,300,000,000 |  |  |
Long-term debt, maturities, repayments of principal in 2023 | 1,700,000,000 |  |  |
Long-term debt, maturities, repayments of principal in 2024 | 1,400,000,000 |  |  |
Long-term debt, maturities, repayments of principal in 2025 | 2,500,000,000 |  |  |
Available borrowing capacity under credit facility | $ 6,000,000,000.0 |  |  |
Operating lease, weighted average remaining lease term | 8 years | 7 years 4 months 24 days |  |
Operating lease, cost | $ 346,000,000 | $ 339,000,000 |  |
Operating lease, payments | 340,000,000 | 281,000,000 |  |
Lessee, operating lease, lease not yet commenced, amount | 475,000,000 |  |  |
Right-of-use asset obtained in exchange for operating lease liability | 495,000,000 | 129,000,000 |  |
Operating leases, rent expense |  |  | $ 322,000,000 |
Other Variable Rate Debt |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt | $ 294,000,000 | $ 147,000,000 |  |
Effective interest rate (as percent) | 0.45% | 2.54% |  |
6.30% debentures due 2026 |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Long-term debt | $ 135,000,000 | $ 135,000,000 |  |
Stated interest rate (as percent) | 6.30% |  |  |
0.75% notes due 2026 |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Stated interest rate (as percent) |  |  |  | 0.75%
Face amount of debt |  |  |  | $ 1,000,000,000.0
1.45% notes due 2030 |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Stated interest rate (as percent) |  |  |  | 1.45%
Face amount of debt |  |  |  | $ 1,250,000,000
2.35% notes due 2040 |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Stated interest rate (as percent) |  |  |  | 2.35%
Face amount of debt |  |  |  | $ 1,000,000,000.0
2.45% notes due 2050 |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Stated interest rate (as percent) |  |  |  | 2.45%
Face amount of debt |  |  |  | $ 1,250,000,000
Notes Subject To Repayment At Option Of Holder |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Loans payable and current portion of long-term debt | $ 73,000,000 | 226,000,000 |  |
Commercial paper |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Loans payable and current portion of long-term debt | $ 4,000,000,000.0 | $ 1,400,000,000 |  |
Weighted-average interest rate of commercial paper (as percent) | 0.79% | 2.23% |  |
Buildings |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Operating lease, weighted average remaining lease term | 8 years |  |  |
Lessee, operating lease, renewal term | 4 years |  |  |
Vehicles |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Operating lease, weighted average remaining lease term | 4 years |  |  |

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