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Investments (Tables) | 12 Months Ended
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Dec. 31, 2021
Investments [Abstract] |
Amortized Cost and Fair Value of the Trust Investments | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Amortized Cost | ​ | Fair Value
​ | ​ | Longest | ​ | December 31,
(in millions) |  | Maturity Date |  | 2021 |  | 2020 |  | 2021 |  | 2020
Stocks |  | — |  | ​ | * |  | ​ | * | ​ | $ | 1,972 | ​ | $ | 1,908
Municipal bonds |  | 2057 |  | ​ | 875 |  | ​ | 1,013 | ​ |  | 1,033 | ​ |  | 1,218
U.S. government and agency securities |  | 2067 |  | ​ | 1,095 |  | ​ | 740 | ​ |  | 1,212 | ​ |  | 864
Corporate bonds |  | 2070 |  | ​ | 386 |  | ​ | 460 | ​ |  | 446 | ​ |  | 550
Short-term investments and receivables/payables1 |  | One-year |  | ​ | 199 |  | ​ | 281 | ​ |  | 207 | ​ |  | 293
Total |  |  | ​ | $ | 2,555 | ​ | $ | 2,494 | ​ | $ | 4,870 | ​ | $ | 4,833
​
* | Not applicable
1 | Short-term investments include $37 million and $138 million of repurchase agreements payable by financial institutions which earn interest, are fully secured by U.S. Treasury securities and mature by January 3, 2022 and January 4, 2021 as of December 31, 2021 and 2020, respectively.
Summary of gains and losses | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | December 31,
(in millions) |  | 2021 |  | 2020 |  | 2019
Gross realized gains | ​ | $ | 339 | ​ | $ | 255 | ​ | $ | 87
Gross realized losses | ​ |  | (24) | ​ |  | (6) | ​ |  | (2)
Net unrealized gains for equity securities | ​ |  | 103 | ​ |  | 176 | ​ |  | 343

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- Definition Tabular disclosure of realized and unrealized gain (loss) on investment in security. + References No definition available. + Details
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- Definition Tabular disclosure of the reconciliation of available-for-sale securities from cost basis to fair value. + References No definition available. + Details
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