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Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - USD ($) $ in Thousands |  | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jan. 04, 2021 | Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Dec. 31, 2020 | Dec. 31, 2019
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |  |  |
Transfer to active union medical account | $ 106,000 |  |  |  |  |  |
APS |  |  |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |  |  |
Fair Value |  | $ 1,581,524 |  | $ 1,581,524 |  | $ 1,392,944 |
Total Unrealized Gains |  |  |  | 541,701 |  | 468,247 |
Total Unrealized Losses |  |  |  | (1,357) |  | (398) |
Amortized cost |  | 815,000 |  | 815,000 |  | 687,000 |
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |  |  |
Realized gains |  | 1,406 | $ 4,500 | 4,374 | $ 7,813 |  |
Realized losses |  | (1,146) | (1,621) | (5,294) | (3,848) |  |
Proceeds from the sale of securities |  | 207,864 | 196,772 | 587,842 | 391,859 |  |
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |  |  |
Employee medical claims amount |  |  |  |  |  | 14,000 | $ 15,000
APS | Equity securities |  |  |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |  |  |
Equity securities |  | 745,489 |  | 745,489 |  | 677,188 |
Total Unrealized Gains |  |  |  | 510,432 |  | 421,666 |
Total Unrealized Losses |  |  |  | 0 |  | 0 |
APS | Available for sale-fixed income securities |  |  |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |  |  |
Fair Value |  | 844,589 |  | 844,589 |  | 733,080 |
Total Unrealized Gains |  |  |  | 31,269 |  | 46,581 |
Total Unrealized Losses |  |  |  | (1,357) |  | (398) |
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |  |  |
Less than one year |  | 94,537 |  | 94,537 |  |  |
1 year – 5 years |  | 331,662 |  | 331,662 |  |  |
5 years – 10 years |  | 195,426 |  | 195,426 |  |  |
Greater than 10 years |  | 222,964 |  | 222,964 |  |  |
Total |  | 844,589 |  | 844,589 |  |  |
APS | Other |  |  |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |  |  |
Fair Value |  | (8,554) |  | (8,554) |  | (17,324) |
Total Unrealized Gains |  |  |  | 0 |  | 0 |
Total Unrealized Losses |  |  |  | 0 |  | 0 |
Nuclear Decommissioning Trust | APS |  |  |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |  |  |
Fair Value |  | 1,223,088 |  | 1,223,088 |  | 1,138,435 |
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |  |  |
Realized gains |  | 1,406 | 4,500 | 4,374 | 7,813 |  |
Realized losses |  | (1,146) | (1,621) | (5,294) | (3,848) |  |
Proceeds from the sale of securities |  | 190,340 | 176,942 | 425,068 | 355,138 |  |
Nuclear Decommissioning Trust | APS | Equity securities |  |  |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |  |  |
Equity securities |  | 725,585 |  | 725,585 |  | 639,851 |
Nuclear Decommissioning Trust | APS | Available for sale-fixed income securities |  |  |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |  |  |
Fair Value |  | 507,009 |  | 507,009 |  | 516,412 |
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |  |  |
Less than one year |  | 28,151 |  | 28,151 |  |  |
1 year – 5 years |  | 133,054 |  | 133,054 |  |  |
5 years – 10 years |  | 131,462 |  | 131,462 |  |  |
Greater than 10 years |  | 214,342 |  | 214,342 |  |  |
Total |  | 507,009 |  | 507,009 |  |  |
Nuclear Decommissioning Trust | APS | Other |  |  |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |  |  |
Fair Value |  | (9,506) |  | (9,506) |  | (17,828) |
Other Special Use Funds | APS |  |  |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |  |  |
Fair Value |  | 358,436 |  | 358,436 |  | 254,509 |
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |  |  |
Realized gains |  | 0 | 0 | 0 | 0 |  |
Realized losses |  | 0 | 0 | 0 | 0 |  |
Proceeds from the sale of securities |  | 17,524 | $ 19,830 | 162,774 | $ 36,721 |  |
Other Special Use Funds | APS | Equity securities |  |  |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |  |  |
Equity securities |  | 19,904 |  | 19,904 |  | 37,337 |
Other Special Use Funds | APS | Available for sale-fixed income securities |  |  |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |  |  |
Fair Value |  | 337,580 |  | 337,580 |  | 216,668 |
Other Special Use Funds | APS | Other |  |  |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |  |  |
Fair Value |  | 952 |  | 952 |  | $ 504 |
Coal Reclamation Escrow Account | APS | Available for sale-fixed income securities |  |  |  |  |  |  |
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |  |  |
Less than one year |  | 26,123 |  | 26,123 |  |  |
1 year – 5 years |  | 35,880 |  | 35,880 |  |  |
5 years – 10 years |  | 2,676 |  | 2,676 |  |  |
Greater than 10 years |  | 8,622 |  | 8,622 |  |  |
Total |  | 73,301 |  | 73,301 |  |  |
Active union medical trust | APS | Available for sale-fixed income securities |  |  |  |  |  |  |
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |  |  |
Less than one year |  | 40,263 |  | 40,263 |  |  |
1 year – 5 years |  | 162,728 |  | 162,728 |  |  |
5 years – 10 years |  | 61,288 |  | 61,288 |  |  |
Greater than 10 years |  | 0 |  | 0 |  |  |
Total |  | $ 264,279 |  | $ 264,279 |  |  |

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