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Nuclear Decommissioning Trusts | 12 Months Ended
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Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract] |
Nuclear Decommissioning Trusts | Nuclear Decommissioning Trusts To fund the costs APS expects to incur to decommission Palo Verde, APS established external decommissioning trusts in accordance with NRC regulations. Third-party investment managers are authorized to buy and sell securities per stated investment guidelines. The trust funds are invested in fixed income securities and equity securities. APS classifies investments in decommissioning trust funds as available for sale. As a result, we record the decommissioning trust funds at their fair value on our Consolidated Balance Sheets. See Note 13 for a discussion of how fair value is determined and the classification of the nuclear decommissioning trust investments within the fair value hierarchy. Because of the ability of APS to recover decommissioning costs in rates and in accordance with the regulatory treatment for decommissioning trust funds, we have deferred realized and unrealized gains and losses (including other-than-temporary impairments on investment securities) in other regulatory liabilities. The following table includes the unrealized gains and losses based on the original cost of the investment and summarizes the fair value of APS’s nuclear decommissioning trust fund assets at December 31, 2016 and December 31, 2015 (dollars in thousands):
| Fair Value |  | Total Unrealized Gains |  | Total Unrealized Losses
December 31, 2016 |  |  |  |  |  |  |  |
Equity securities | $ | 353,261 |  |  | $ | 188,091 |  |  | $ | — |
Fixed income securities | 425,530 |  |  | 9,820 |  |  | (4,962 | )
Net receivables (a) | 795 |  |  | — |  |  | — |
Total | $ | 779,586 |  |  | $ | 197,911 |  |  | $ | (4,962 | )
| Fair Value |  | Total Unrealized Gains |  | Total Unrealized Losses
December 31, 2015 |  |  |  |  |  |  |  |
Equity securities | $ | 314,957 |  |  | $ | 157,098 |  |  | $ | (115 | )
Fixed income securities | 420,574 |  |  | 11,955 |  |  | (2,645 | )
Net payables (a) | (335 | ) |  | — |  |  | — |
Total | $ | 735,196 |  |  | $ | 169,053 |  |  | $ | (2,760 | )
(a) | Net receivables/(payables) relate to pending purchases and sales of securities.
The costs of securities sold are determined on the basis of specific identification. The following table sets forth approximate gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds (dollars in thousands):
| Year Ended December 31,
| 2016 |  | 2015 |  | 2014
Realized gains | $ | 11,213 |  |  | $ | 5,189 |  |  | $ | 4,725 |
Realized losses | (10,106 | ) |  | (6,225 | ) |  | (4,525 | )
Proceeds from the sale of securities (a) | 633,410 |  |  | 478,813 |  |  | 356,195 |
(a) | Proceeds are reinvested in the trust.
The fair value of fixed income securities, summarized by contractual maturities, at December 31, 2016 is as follows (dollars in thousands):
| Fair Value
Less than one year | $ | 13,063 |
1 year – 5 years | 119,292 |
5 years – 10 years | 105,612 |
Greater than 10 years | 187,563 |
Total | $ | 425,530 |

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