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Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - APS - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Dec. 31, 2018
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | $ 1,192,006 |  | $ 1,192,006 |  | $ 1,087,235
Total Unrealized Gains | 315,933 |  | 315,933 |  | 230,781
Total Unrealized Losses | (696) |  | (696) |  | (7,237)
Amortized cost | 672,000 |  | 672,000 |  | 635,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |
Realized gains | 2,643 | $ 1,484 | 3,746 | $ 2,299 |
Realized losses | (1,700) | (2,978) | (3,105) | (5,025) |
Proceeds from the sale of securities | 130,306 | 125,216 | 309,354 | 258,227 |
Equity securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Equity securities | 487,894 |  | 487,894 |  | 447,138
Total Unrealized Gains | 291,295 |  | 291,295 |  | 222,147
Total Unrealized Losses | 0 |  | 0 |  | 459
Available for sale-fixed income securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 696,450 |  | 696,450 |  | 637,356
Total Unrealized Gains | 24,638 |  | 24,638 |  | 8,634
Total Unrealized Losses | (696) |  | (696) |  | (6,778)
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Less than one year | 99,845 |  | 99,845 |  |
1 year – 5 years | 284,763 |  | 284,763 |  |
5 years – 10 years | 115,796 |  | 115,796 |  |
Greater than 10 years | 196,046 |  | 196,046 |  |
Total | 696,450 |  | 696,450 |  |
Other |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 7,662 |  | 7,662 |  | 2,741
Total Unrealized Gains | 0 |  | 0 |  | 0
Total Unrealized Losses | 0 |  | 0 |  | 0
Nuclear Decommissioning Trust |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 950,448 |  | 950,448 |  | 851,134
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |
Realized gains | 2,643 | 1,484 | 3,746 | 2,298 |
Realized losses | (1,700) | (2,978) | (3,105) | (5,025) |
Proceeds from the sale of securities | 93,559 | 122,790 | 216,152 | 253,246 |
Nuclear Decommissioning Trust | Equity securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Equity securities | 474,694 |  | 474,694 |  | 402,008
Nuclear Decommissioning Trust | Available for sale-fixed income securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 469,089 |  | 469,089 |  | 446,978
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Less than one year | 37,373 |  | 37,373 |  |
1 year – 5 years | 126,745 |  | 126,745 |  |
5 years – 10 years | 113,989 |  | 113,989 |  |
Greater than 10 years | 190,982 |  | 190,982 |  |
Total | 469,089 |  | 469,089 |  |
Nuclear Decommissioning Trust | Other |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 6,665 |  | 6,665 |  | 2,148
Other Special Use Funds |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 241,558 |  | 241,558 |  | 236,101
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |
Realized gains | 0 | 0 | 0 | 1 |
Realized losses | 0 | 0 | 0 | 0 |
Proceeds from the sale of securities | 36,747 | $ 2,426 | 93,202 | $ 4,981 |
Other Special Use Funds | Equity securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Equity securities | 13,200 |  | 13,200 |  | 45,130
Other Special Use Funds | Available for sale-fixed income securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 227,361 |  | 227,361 |  | 190,378
Other Special Use Funds | Other |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 997 |  | 997 |  | $ 593
Coal Reclamation Escrow Account | Available for sale-fixed income securities |  |  |  |  |
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Less than one year | 22,253 |  | 22,253 |  |
1 year – 5 years | 17,449 |  | 17,449 |  |
5 years – 10 years | 1,807 |  | 1,807 |  |
Greater than 10 years | 5,064 |  | 5,064 |  |
Total | 46,573 |  | 46,573 |  |
Active Union Medical Trust | Available for sale-fixed income securities |  |  |  |  |
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Less than one year | 40,219 |  | 40,219 |  |
1 year – 5 years | 140,569 |  | 140,569 |  |
5 years – 10 years | 0 |  | 0 |  |
Greater than 10 years | 0 |  | 0 |  |
Total | $ 180,788 |  | $ 180,788 |  |

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