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Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
APS |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | $ 1,089,981 |  | $ 1,089,981 |  | $ 901,358
Total Unrealized Gains | 261,238 |  | 261,238 |  | 260,160
Total Unrealized Losses | (9,805) |  | (9,805) |  | (2,996)
Amortized cost | 639,000 |  | 639,000 |  | 467,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |
Realized gains | 1,484 | $ 956 | 2,299 | $ 3,323 |
Realized losses | (2,978) | (1,167) | (5,025) | (3,621) |
Proceeds from the sale of securities | 125,216 | 125,810 | 258,227 | 279,457 |
APS | Equity securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 446,724 |  | 446,724 |  |
Total Unrealized Gains | 254,342 |  | 254,342 |  |
Total Unrealized Losses | 1 |  | 1 |  |
Fair Value |  |  |  |  | 425,044
Total Unrealized Gains |  |  |  |  | 248,623
Total Unrealized Losses |  |  |  |  | 0
APS | Available for sale-fixed income securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 641,400 |  | 641,400 |  | 475,716
Total Unrealized Gains | 6,896 |  | 6,896 |  | 11,537
Total Unrealized Losses | (9,804) |  | (9,804) |  | (2,996)
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Less than one year | 48,856 |  | 48,856 |  |
1 year – 5 years | 260,373 |  | 260,373 |  |
5 years – 10 years | 130,314 |  | 130,314 |  |
Greater than 10 years | 201,857 |  | 201,857 |  |
Total | 641,400 |  | 641,400 |  |
APS | Other |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 1,857 |  | 1,857 |  | 598
Total Unrealized Gains | 0 |  | 0 |  | 0
Total Unrealized Losses | 0 |  | 0 |  | 0
Nuclear Decommissioning Trusts | APS |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 873,643 |  | 873,643 |  | 871,000
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |
Realized gains | 1,484 | 939 | 2,298 | 3,306 |
Realized losses | (2,978) | (1,159) | (5,025) | (3,612) |
Proceeds from the sale of securities | 122,790 | 124,238 | 253,246 | 275,364 |
Nuclear Decommissioning Trusts | APS | Equity securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 432,615 |  | 432,615 |  |
Fair Value |  |  |  |  | 424,614
Nuclear Decommissioning Trusts | APS | Available for sale-fixed income securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 440,403 |  | 440,403 |  |
Fair Value |  |  |  |  | 446,277
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Less than one year | 18,415 |  | 18,415 |  |
1 year – 5 years | 105,470 |  | 105,470 |  |
5 years – 10 years | 127,599 |  | 127,599 |  |
Greater than 10 years | 188,919 |  | 188,919 |  |
Total | 440,403 |  | 440,403 |  |
Nuclear Decommissioning Trusts | APS | Other |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 625 |  | 625 |  | 109
Other Special Use Funds | APS |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 216,338 |  | 216,338 |  | 30,358
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds |  |  |  |  |
Realized gains | 0 | 17 | 1 | 17 |
Realized losses | 0 | (8) | 0 | (9) |
Proceeds from the sale of securities | 2,426 | $ 1,572 | 4,981 | $ 4,093 |
Other Special Use Funds | APS | Equity securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 14,109 |  | 14,109 |  |
Fair Value |  |  |  |  | 430
Other Special Use Funds | APS | Available for sale-fixed income securities |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 200,997 |  | 200,997 |  |
Fair Value |  |  |  |  | 29,439
Other Special Use Funds | APS | Other |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 1,232 |  | 1,232 |  | 489
Coal Reclamation Escrow Accounts | APS | Available for sale-fixed income securities |  |  |  |  |
Fair value of fixed income securities, summarized by contractual maturities |  |  |  |  |
Less than one year | 0 |  | 0 |  |
1 year – 5 years | 11,706 |  | 11,706 |  |
5 years – 10 years | 2,715 |  | 2,715 |  |
Greater than 10 years | 12,938 |  | 12,938 |  |
Total | 27,359 |  | 27,359 |  |
Coal Reclamation Escrow Accounts | 4CA |  |  |  |  |
Nuclear decommissioning trust fund assets |  |  |  |  |
Fair Value | 2,000 |  | 2,000 |  | $ 2,000
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 | 30,441 |  | 30,441 |  |
1 year – 5 years | 143,197 |  | 143,197 |  |
5 years – 10 years | 0 |  | 0 |  |
Greater than 10 years | 0 |  | 0 |  |
Total | $ 173,638 |  | $ 173,638 |  |

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