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Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - USD ($) $ in Thousands |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jan. 04, 2021 | Mar. 31, 2021 | Mar. 31, 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 |  | $ 1,517,205 |  | $ 1,392,944 |
Total Unrealized Gains |  | 484,780 |  | 468,247 |
Total Unrealized Losses |  | (3,203) |  | (398) |
Amortized cost |  | 818,000 |  | 687,000 |
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |
Realized gains |  | 2,968 | $ 3,313 |  |
Realized losses |  | (4,148) | (2,227) |  |
Proceeds from the sale of securities |  | 379,978 | 195,087 |  |
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 |  | 689,570 |  | 677,188 |
Total Unrealized Gains |  | 457,442 |  | 421,666 |
Total Unrealized Losses |  | 0 |  | 0 |
APS | Available for sale-fixed income securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value |  | 841,772 |  | 733,080 |
Total Unrealized Gains |  | 27,338 |  | 46,581 |
Total Unrealized Losses |  | (3,203) |  | (398) |
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Less than one year |  | 91,776 |  |  |
1 year – 5 years |  | 342,607 |  |  |
5 years – 10 years |  | 203,664 |  |  |
Greater than 10 years |  | 203,725 |  |  |
Total |  | 841,772 |  |  |
APS | Other |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Other |  | (14,137) |  | (17,324) |
Total Unrealized Gains |  | 0 |  | 0 |
Total Unrealized Losses |  | 0 |  | 0 |
Nuclear Decommissioning Trust | APS |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value |  | 1,159,699 |  | 1,138,435 |
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |
Realized gains |  | 2,968 | 3,313 |  |
Realized losses |  | (4,148) | (2,227) |  |
Proceeds from the sale of securities |  | 234,728 | 178,196 |  |
Nuclear Decommissioning Trust | APS | Equity securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Equity securities |  | 670,359 |  | 639,851 |
Nuclear Decommissioning Trust | APS | Available for sale-fixed income securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value |  | 504,878 |  | 516,412 |
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Less than one year |  | 25,048 |  |  |
1 year – 5 years |  | 147,347 |  |  |
5 years – 10 years |  | 137,479 |  |  |
Greater than 10 years |  | 195,004 |  |  |
Total |  | 504,878 |  |  |
Nuclear Decommissioning Trust | APS | Other |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Other |  | (15,538) |  | (17,828) |
Other Special Use Funds | APS |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value |  | 357,506 |  | 254,509 |
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |
Realized gains |  | 0 | 0 |  |
Realized losses |  | 0 | 0 |  |
Proceeds from the sale of securities |  | 145,250 | $ 16,891 |  |
Other Special Use Funds | APS | Equity securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Equity securities |  | 19,211 |  | 37,337 |
Other Special Use Funds | APS | Available for sale-fixed income securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value |  | 336,894 |  | 216,668 |
Other Special Use Funds | APS | Other |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Other |  | 1,401 |  | $ 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 |  | 26,259 |  |  |
1 year – 5 years |  | 34,936 |  |  |
5 years – 10 years |  | 2,708 |  |  |
Greater than 10 years |  | 8,721 |  |  |
Total |  | 72,624 |  |  |
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 |  | 40,469 |  |  |
1 year – 5 years |  | 160,324 |  |  |
5 years – 10 years |  | 63,477 |  |  |
Greater than 10 years |  | 0 |  |  |
Total |  | $ 264,270 |  |  |

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