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Investments (Tables) | 12 Months Ended
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Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract] |
Fair value of APS's nuclear decommissioning trust fund assets | 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, 2017 and December 31, 2016 (dollars in thousands):
| December 31, 2017 |  | December 31, 2016
| Fair Value |  | Total Unrealized Gains |  | Total Unrealized Losses |  | Fair Value |  | Total Unrealized Gains |  | Total Unrealized Losses
Equity securities | $ | 417,390 |  |  | $ | 248,623 |  |  | $ | — |  |  | $ | 353,261 |  |  | $ | 188,091 |  |  | $ | — |
Fixed income securities | 446,277 |  |  | 11,537 |  |  | (2,996 | ) |  | 425,530 |  |  | 9,820 |  |  | (4,962 | )
Cash and cash equivalents | 7,224 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Net receivables (a) | 109 |  |  | — |  |  | — |  |  | 795 |  |  | — |  |  | — |
Total | $ | 871,000 |  |  | $ | 260,160 |  |  | $ | (2,996 | ) |  | $ | 779,586 |  |  | $ | 197,911 |  |  | $ | (4,962 | )
(a) | Net receivables/(payables) relate to pending purchases and sales of securities.
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds | 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):
| Nuclear Decommissioning
| Year Ended December 31,
| 2017 |  | 2016 |  | 2015
Realized gains | $ | 21,813 |  |  | $ | 11,213 |  |  | $ | 5,189 |
Realized losses | (13,146 | ) |  | (10,106 | ) |  | (6,225 | )
Proceeds from the sale of securities (a) | 542,246 |  |  | 633,410 |  |  | 478,813 |
(a) | Proceeds are reinvested in the trust/account.
Fair value of fixed income securities, summarized by contractual maturities | The fair value of fixed income securities, summarized by contractual maturities, at December 31, 2017 is as follows (dollars in thousands):
| Nuclear Decommissioning Trusts |  | Escrow Accounts |  | Total
Less than one year | $ | 24,668 |  |  | $ | 455 |  |  | $ | 25,123 |
1 year – 5 years | 100,289 |  |  | 2,494 |  |  | 102,783 |
5 years – 10 years | 129,239 |  |  | 8,615 |  |  | 137,854 |
Greater than 10 years | 192,081 |  |  | 20,453 |  |  | 212,534 |
Total | $ | 446,277 |  |  | $ | 32,017 |  |  | $ | 478,294 |

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- Definition Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62557-112803 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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- Definition Tabular disclosure of the proceeds from sales of available-for-sale securities and the gross realized gains and gross realized losses that have been included in earnings as a result of those sales. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 + Details
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