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Investments in Nuclear Decommissioning Trusts and Other Special Use Funds (Tables) | 3 Months Ended
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Mar. 31, 2018
Investments, Debt and Equity Securities [Abstract] |
Fair value of APS's nuclear decommissioning trust fund assets | The following tables present the unrealized gains and losses based on the original cost of the investment and summarizes the fair value of APS's nuclear decommissioning trust and other special use fund assets at March 31, 2018 and December 31, 2017 (dollars in thousands):
| March 31, 2018
| Fair Value |  | Total Unrealized Gains |  | Total Unrealized Losses
Investment Type: | Nuclear Decommissioning Trusts |  | Other Special Use Funds |  | Total |  |
Equity securities | $ | 420,265 |  |  | $ | 14,641 |  |  | $ | 434,906 |  |  | $ | 242,784 |  |  | $ | (61 | )
Available for sale-fixed income securities | 441,241 |  |  | 199,701 |  |  | 640,942 |  | (a) | 7,701 |  |  | (8,545 | )
Other | (67 | ) |  | 1,458 |  |  | 1,391 |  | (b) | — |  |  | — |
Total | $ | 861,439 |  |  | $ | 215,800 |  |  | $ | 1,077,239 |  |  | $ | 250,485 |  |  | $ | (8,606 | )
(a) | As of March 31, 2018 the amortized cost basis of these available-for-sale investments is $642 million.
(b) | Represents net pending securities sales and purchases.
| December 31, 2017
| Fair Value |  | Total Unrealized Gains |  | Total Unrealized Losses
Investment Type: | Nuclear Decommissioning Trusts |  | Other Special Use Funds |  | Total |  |
Equity securities | $ | 424,614 |  |  | $ | 430 |  |  | $ | 425,044 |  |  | $ | 248,623 |  |  | $ | — |
Available for sale-fixed income securities | 446,277 |  |  | 29,439 |  |  | 475,716 |  | (a) | 11,537 |  |  | (2,996 | )
Other | 109 |  |  | 489 |  |  | 598 |  | (b) | — |  |  | — |
Total | $ | 871,000 |  |  | $ | 30,358 |  |  | $ | 901,358 |  |  | $ | 260,160 |  |  | $ | (2,996 | )
(a) | As of December 31, 2017 the amortized cost basis of these available-for-sale investments is $467 million.
(b) | Represents net pending securities sales and purchases.
Realized gains and losses and proceeds from the sale of securities by the nuclear decommissioning trust funds | The following table sets forth APS's realized gains and losses relating to the sale and maturity of available-for-sale debt securities and equity securities, and the proceeds from the sale and maturity of these investment securities for the three months ended March 31, 2018 and March 31, 2017 (dollars in thousands):
| Three Months Ended March 31, 2018 |  | Three Months Ended March 31, 2017
| Nuclear Decommissioning Trusts |  | Other Special Use Funds |  | Total |  | Nuclear Decommissioning Trusts |  | Other Special Use Funds |  | Total
Realized gains | $ | 814 |  |  | $ | 1 |  |  | $ | 815 |  |  | $ | 2,367 |  |  | $ | — |  |  | $ | 2,367 |
Realized losses | (2,047 | ) |  | — |  |  | (2,047 | ) |  | (2,453 | ) |  | (1 | ) |  | (2,454 | )
Proceeds from the sale of securities (a) | 130,456 |  |  | 2,555 |  |  | 133,011 |  |  | 151,126 |  |  | 2,521 |  |  | 153,647 |
(a) | Proceeds are reinvested in the trust or escrow accounts.
Fair value of fixed income securities, summarized by contractual maturities | The fair value of APS's fixed income securities, summarized by contractual maturities, at March 31, 2018, is as follows (dollars in thousands):
| Nuclear Decommissioning Trusts (a) |  | Coal Reclamation Escrow Accounts |  | Active Union Medical Trust |  | Total
Less than one year | $ | 16,148 |  |  | $ | — |  |  | $ | 30,320 |  |  | $ | 46,468 |
1 year – 5 years | 109,466 |  |  | 7,541 |  |  | 143,791 |  |  | 260,798 |
5 years – 10 years | 125,206 |  |  | 2,736 |  |  | — |  |  | 127,942 |
Greater than 10 years | 190,421 |  |  | 15,313 |  |  | — |  |  | 205,734 |
Total | $ | 441,241 |  |  | $ | 25,590 |  |  | $ | 174,111 |  |  | $ | 640,942 |
(a) | Includes certain fixed income investments that are not due at a single maturity date. These investments have been allocated within the table based on the final payment date of the instrument.

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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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