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Investments (Tables) | 3 Months Ended
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Mar. 31, 2021
Regulated Entity, Other Assets, Noncurrent [Abstract] |
Amortized Cost and Fair Value of the Trust Investments | ​
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​ | ​ | ​ | ​ | Amortized Cost | ​ | Fair Value
​ | ​ | Longest | ​ | March 31, | ​ | December 31, | ​ | March 31, | ​ | December 31,
(in millions) |  | Maturity Dates |  | 2021 |  | 2020 |  | 2021 |  | 2020
Stocks |  | — |  | ​ | N/A |  | ​ | N/A | ​ | $ | 1,896 | ​ | $ | 1,908
Municipal bonds |  | 2054 |  | $ | 1,044 |  | $ | 1,013 | ​ |  | 1,212 | ​ |  | 1,218
U.S. government and agency securities |  | 2067 |  | ​ | 683 |  | ​ | 740 | ​ |  | 754 | ​ |  | 864
Corporate bonds |  | 2070 |  | ​ | 451 |  | ​ | 460 | ​ |  | 519 | ​ |  | 550
Short-term investments and receivables/payables1 |  | One-year |  | ​ | 366 |  | ​ | 281 | ​ |  | 382 | ​ |  | 293
Total |  |  | ​ | $ | 2,544 | ​ | $ | 2,494 | ​ | $ | 4,763 | ​ | $ | 4,833
1 | Short-term investments include $144 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 April 1, 2021 and January 4, 2021 as of March 31, 2021 and December 31, 2020, respectively.
Summary of gains and losses | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three months ended March 31, | ​
(in millions) |  | 2021 |  | 2020 | ​
Gross realized gains | ​ | $ | 111 | ​ | $ | 39 | ​
Gross realized loss | ​ |  | 12 | ​ |  | 2 | ​
Net unrealized gains (losses) for equity securities | ​ |  | 18 | ​ |  | (376) | ​

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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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