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Retirement Plans and Other Benefits - Fair Value of Pinnacle West's Pension Plan (Details) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | $ 832,143 | $ 726,093 |
Fair value of plan assets | 3,318,351 | 2,733,476 | $ 3,057,027
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 852,239 | 690,737 |
Pension Benefits | Significant Other Observable Inputs (Level 2) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 1,633,969 | 1,316,646 |
Other Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 107,473 | 90,955 |
Fair value of plan assets | 837,494 | 723,677 | $ 1,022,371
Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 519,382 | 461,905 |
Other Benefits | Significant Other Observable Inputs (Level 2) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 210,639 | 170,817 |
Cash and cash equivalents | Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 9,370 | 451 |
Cash and cash equivalents | Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 9,370 | 451 |
Cash and cash equivalents | Other Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 2,184 | 93 |
Cash and cash equivalents | Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 2,184 | 93 |
Corporate | Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 1,541,729 | 1,237,744 |
Corporate | Pension Benefits | Significant Other Observable Inputs (Level 2) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 1,541,729 | 1,237,744 |
Corporate | Other Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 202,640 | 163,286 |
Corporate | Other Benefits | Significant Other Observable Inputs (Level 2) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 202,640 | 163,286 |
U.S. Treasury | Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 406,112 | 372,649 |
U.S. Treasury | Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 406,112 | 372,649 |
U.S. Treasury | Other Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 353,650 | 318,017 |
U.S. Treasury | Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 353,650 | 318,017 |
Other fixed income | Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 92,240 | 78,902 |
Other fixed income | Pension Benefits | Significant Other Observable Inputs (Level 2) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 92,240 | 78,902 |
Other fixed income | Other Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 7,999 | 7,531 |
Other fixed income | Other Benefits | Significant Other Observable Inputs (Level 2) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 7,999 | 7,531 |
Common stock equities | Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 250,829 | 196,661 |
Common stock equities | Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 250,829 | 196,661 |
Common stock equities | Other Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 146,316 | 129,199 |
Common stock equities | Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 146,316 | 129,199 |
Mutual funds | Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 185,928 | 120,976 |
Mutual funds | Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 185,928 | 120,976 |
Mutual funds | Other Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 14,351 | 10,963 |
Mutual funds | Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 14,351 | 10,963 |
Equities | Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 392,403 | 272,926 |
Fair value of plan assets | 392,403 | 272,926 |
Equities | Other Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 83,648 | 65,720 |
Fair value of plan assets | 83,648 | 65,720 |
Real estate | Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 171,645 | 165,123 |
Fair value of plan assets | 171,645 | 165,123 |
Real estate | Other Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 19,806 | 19,054 |
Fair value of plan assets | 19,806 | 19,054 |
Fixed income securities | Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 98,065 | 86,483 |
Fair value of plan assets | 98,065 | 86,483 |
Partnerships | Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 103,796 | 125,217 |
Fair value of plan assets | 103,796 | 125,217 |
Short-term investments and other | Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 66,234 | 76,344 |
Fair value of plan assets | 66,234 | 76,344 |
Short-term investments and other | Pension Benefits | Significant Other Observable Inputs (Level 2) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 0 |  |
Short-term investments and other | Other Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 4,019 | 6,181 |
Fair value of plan assets | 6,900 | 9,814 |
Short-term investments and other | Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 2,881 | 3,633 |
Short-term investments and other | Other Benefits | Significant Other Observable Inputs (Level 2) |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | $ 0 | $ 0 |

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