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Retirement Plans and Other Benefits - Fair Value of Pinnacle West's Pension Plan (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | $ 726,093 | $ 903,085 |
Fair value of plan assets | 2,733,476 | 3,057,027 | $ 2,675,357
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 | 690,737 | 686,941 |
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 | 1,316,646 | 1,467,001 |
Other Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 90,955 | 99,910 |
Fair value of plan assets | 723,677 | 1,022,371 | $ 882,651
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 | 461,905 | 583,796 |
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 | 170,817 | 338,665 |
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 | 1,237,744 | 1,365,194 |
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 | 1,237,744 | 1,365,194 |
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 | 163,286 | 306,008 |
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 | 163,286 | 306,008 |
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 | 372,649 | 221,291 |
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 | 372,649 | 221,291 |
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 | 318,017 | 336,963 |
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 | 318,017 | 336,963 |
Other | Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 78,902 | 100,599 |
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 | 78,902 | 100,599 |
Other | Other Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 7,531 | 32,508 |
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 | 7,531 | 32,508 |
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 | 196,661 | 228,088 |
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 | 196,661 | 228,088 |
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 | 129,199 | 196,153 |
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 | 129,199 | 196,153 |
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 | 120,976 | 233,732 |
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 | 120,976 | 233,732 |
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 | 10,963 | 39,269 |
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 | 10,963 | 39,269 |
Equities | Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 272,926 | 408,763 |
Fair value of plan assets | 272,926 | 408,763 |
Equities | Other Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 65,720 | 75,310 |
Fair value of plan assets | 65,720 | 75,310 |
Real estate | Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 165,123 | 171,569 |
Fair value of plan assets | 165,123 | 171,569 |
Real estate | Other Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 19,054 | 15,422 |
Fair value of plan assets | 19,054 | 15,422 |
Fixed income securities | Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 86,483 | 90,869 |
Fair value of plan assets | 86,483 | 90,869 |
Partnerships | Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 125,217 | 133,379 |
Fair value of plan assets | 125,217 | 133,379 |
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 | 76,344 | 98,505 |
Fair value of plan assets | 76,344 | 99,713 |
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 | 1,208 |
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 | 6,181 | 9,178 |
Fair value of plan assets | 9,814 | 20,595 |
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 | 3,633 |  |
Other |  | 11,268 |
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 |  |
Other |  | 149 |
Defined Benefit Plan, Cash and Cash Equivalents [Member] | Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 451 | 3,830 |
Defined Benefit Plan, Cash and Cash Equivalents [Member] | 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 | 451 | 3,830 |
Defined Benefit Plan, Cash and Cash Equivalents [Member] | Other Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 93 | 143 |
Defined Benefit Plan, Cash and Cash Equivalents [Member] | 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 | $ 93 | $ 143 |

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