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Investments (Tables) | 9 Months Ended
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Sep. 30, 2021
Investments [Abstract] |
Amortized Cost and Fair Value of the Trust Investments | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Amortized Cost | ​ | Fair Value
​ | ​ | Longest | ​ | September 30, | ​ | December 31, | ​ | September 30, | ​ | December 31,
(in millions) |  | Maturity Dates |  | 2021 |  | 2020 |  | 2021 |  | 2020
Stocks |  | — |  | ​ | N/A |  | ​ | N/A | ​ | $ | 1,819 | ​ | $ | 1,908
Municipal bonds |  | 2054 |  | $ | 836 |  | $ | 1,013 | ​ |  | 990 | ​ |  | 1,218
U.S. government and agency securities |  | 2067 |  | ​ | 1,129 |  | ​ | 740 | ​ |  | 1,238 | ​ |  | 864
Corporate bonds |  | 2070 |  | ​ | 394 |  | ​ | 460 | ​ |  | 459 | ​ |  | 550
Short-term investments and receivables/payables1 |  | One-year |  | ​ | 252 |  | ​ | 281 | ​ |  | 263 | ​ |  | 293
Total |  |  | ​ | $ | 2,611 | ​ | $ | 2,494 | ​ | $ | 4,769 | ​ | $ | 4,833
1 | Short-term investments include $59 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 October 1, 2021 and January 4, 2021 as of September 30, 2021 and December 31, 2020, respectively.
Summary of gains and losses | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three months ended September 30, | ​ | Nine months ended September 30,
(in millions) |  | 2021 |  | 2020 |  | 2021 |  | 2020
Gross realized gains | ​ | $ | 77 | ​ | $ | 51 | ​ | $ | 328 | ​ | $ | 165
Gross realized losses | ​ |  | (5) | ​ |  | (2) | ​ |  | (21) | ​ |  | (5)
Net unrealized (losses) gains for equity securities | ​ |  | (74) | ​ |  | 110 | ​ |  | (41) | ​ |  | 5

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