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Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - APS - USD ($) $ in Thousands | 3 Months Ended |
--- | --- | ---
Mar. 31, 2020 | Mar. 31, 2019 | Dec. 31, 2019
Nuclear decommissioning trust fund assets |  |  |
Fair Value | $ 1,173,149 |  | $ 1,255,870
Total Unrealized Gains | 276,150 |  | 363,476
Total Unrealized Losses | (4,728) |  | (669)
Amortized cost | 697,000 |  | 691,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |
Realized gains | 3,313 | $ 1,103 |
Realized losses | (2,227) | (1,405) |
Proceeds from the sale of securities | 195,087 | 179,048 |
Equity securities |  |  |
Nuclear decommissioning trust fund assets |  |  |
Equity securities | 434,667 |  | 536,858
Total Unrealized Gains | 234,695 |  | 337,681
Total Unrealized Losses | (1,201) |  | 0
Available for sale-fixed income securities |  |  |
Nuclear decommissioning trust fund assets |  |  |
Fair Value | 735,061 |  | 716,137
Total Unrealized Gains | 41,455 |  | 25,795
Total Unrealized Losses | (3,527) |  | (669)
Fair value of fixed income securities, summarized by contractual maturities |  |  |
Less than one year | 90,281 |  |
1 year – 5 years | 299,561 |  |
5 years – 10 years | 112,834 |  |
Greater than 10 years | 232,385 |  |
Total | 735,061 |  |
Other |  |  |
Nuclear decommissioning trust fund assets |  |  |
Fair Value | 3,421 |  | 2,875
Total Unrealized Gains | 0 |  | 0
Total Unrealized Losses | 0 |  | 0
Nuclear Decommissioning Trusts [Member] |  |  |
Nuclear decommissioning trust fund assets |  |  |
Fair Value | 920,426 |  | 1,010,775
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |
Realized gains | 3,313 | 1,103 |
Realized losses | (2,227) | (1,405) |
Proceeds from the sale of securities | 178,196 | 122,593 |
Nuclear Decommissioning Trusts [Member] | Equity securities |  |  |
Nuclear decommissioning trust fund assets |  |  |
Equity securities | 427,915 |  | 529,716
Nuclear Decommissioning Trusts [Member] | Available for sale-fixed income securities |  |  |
Nuclear decommissioning trust fund assets |  |  |
Fair Value | 490,421 |  | 478,658
Fair value of fixed income securities, summarized by contractual maturities |  |  |
Less than one year | 11,911 |  |
1 year – 5 years | 137,509 |  |
5 years – 10 years | 112,834 |  |
Greater than 10 years | 228,167 |  |
Total | 490,421 |  |
Nuclear Decommissioning Trusts [Member] | Other |  |  |
Nuclear decommissioning trust fund assets |  |  |
Fair Value | 2,090 |  | 2,401
Other Special Use Funds [Member] |  |  |
Nuclear decommissioning trust fund assets |  |  |
Fair Value | 252,723 |  | 245,095
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 | 16,891 | $ 56,455 |
Other Special Use Funds [Member] | Equity securities |  |  |
Nuclear decommissioning trust fund assets |  |  |
Equity securities | 6,752 |  | 7,142
Other Special Use Funds [Member] | Available for sale-fixed income securities |  |  |
Nuclear decommissioning trust fund assets |  |  |
Fair Value | 244,640 |  | 237,479
Other Special Use Funds [Member] | Other |  |  |
Nuclear decommissioning trust fund assets |  |  |
Fair Value | 1,331 |  | $ 474
Coal Reclamation Escrow Accounts [Member] | Available for sale-fixed income securities |  |  |
Fair value of fixed income securities, summarized by contractual maturities |  |  |
Less than one year | 37,498 |  |
1 year – 5 years | 18,487 |  |
5 years – 10 years | 0 |  |
Greater than 10 years | 4,218 |  |
Total | 60,203 |  |
Active union medical trust | Available for sale-fixed income securities |  |  |
Fair value of fixed income securities, summarized by contractual maturities |  |  |
Less than one year | 40,872 |  |
1 year – 5 years | 143,565 |  |
5 years – 10 years | 0 |  |
Greater than 10 years | 0 |  |
Total | $ 184,437 |  |

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