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Investment in Affiliates and Related-Party Transactions - Narrative (Details) shares in Millions, $ in Millions |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | ---
Oct. 17, 2019 USD ($) shares | Dec. 31, 2020 USD ($) shares | Dec. 31, 2020 USD ($) shares | Dec. 31, 2019 USD ($) shares | Dec. 31, 2018 USD ($) shares | Dec. 25, 2015 note
Related Party Transaction [Line Items] |  |  |  |  |  |
Number of shares repurchased (in shares) | shares |  |  | 13.0 | 48.0 | 72.0 |
Amount of shares repurchased |  |  | $ 867 | $ 3,373 | $ 4,671 |
January 2019 program |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Number of shares repurchased (in shares) | shares |  | 1.1 |  |  |  |
Conrail |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Payable balance |  | $ 217 | $ 217 |  |  |
Corporate Joint Venture | Conrail |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Ownership percentage |  | 42.00% | 42.00% |  |  |
Voting interest percentage |  | 50.00% | 50.00% |  |  |
Difference between carrying amount and underlying equity |  | $ 335 | $ 335 |  |  |
Promissory notes executed | note |  |  |  |  |  | 2
Interest expense |  |  | 6 | $ 6 | $ 6 |
Corporate Joint Venture | Conrail | Long-term Debt | 2.89% Promissory Note Due October 2044 |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Previous notes converted in non-cash transaction |  |  | $ 224 |  |  |
Note interest rate |  |  | 2.89% |  |  |
Corporate Joint Venture | Conrail | Long-term Debt | 1.31% Promissory Note Due December 2050 |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Note interest rate |  |  | 1.31% |  |  |
New notes issued in non-cash transaction |  |  | $ 441 |  |  |
Affiliated Entity | TTX |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Ownership percentage |  | 20.00% | 20.00% |  |  |
Affiliated Entity | MR Argent Advisor LLC | January 2019 program |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |
Number of shares repurchased (in shares) | shares | 4.7 |  |  |  |  |
Amount of shares repurchased | $ 319 |  |  |  |  |

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