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Retirement Plans and Other Benefits - Fair Value of Pinnacle West's Pension Plan (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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Pension Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | $ 3,057,027 | $ 2,675,357 | $ 2,542,774
Pension Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 903,085 | 749,541 |
Fair value of plan assets | 3,057,027 | 2,675,357 |
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 686,941 | 613,193 |
Pension Benefits | Significant Other Observable Inputs (Level 2) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 1,467,001 | 1,312,623 |
Other Benefits |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 1,022,371 | 882,651 | $ 833,017
Other Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 99,910 | 123,234 |
Fair value of plan assets | 1,022,371 | 882,651 |
Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 583,796 | 456,718 |
Other Benefits | Significant Other Observable Inputs (Level 2) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 338,665 | 302,699 |
Cash and cash equivalents | Pension Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 3,830 | 13,995 |
Cash and cash equivalents | Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 3,830 | 13,995 |
Cash and cash equivalents | Other Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 143 | 304 |
Cash and cash equivalents | Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 143 | 304 |
Corporate | Pension Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 1,365,194 | 1,210,453 |
Corporate | Pension Benefits | Significant Other Observable Inputs (Level 2) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 1,365,194 | 1,210,453 |
Corporate | Other Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 306,008 | 268,193 |
Corporate | Other Benefits | Significant Other Observable Inputs (Level 2) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 306,008 | 268,193 |
U.S. Treasury | Pension Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 221,291 | 112,583 |
U.S. Treasury | Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 221,291 | 112,583 |
U.S. Treasury | Other Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 336,963 | 145,255 |
U.S. Treasury | Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 336,963 | 145,255 |
Other | Pension Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 100,599 | 102,170 |
Other | Pension Benefits | Significant Other Observable Inputs (Level 2) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 100,599 | 102,170 |
Other | Other Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 32,508 | 34,506 |
Other | Other Benefits | Significant Other Observable Inputs (Level 2) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 32,508 | 34,506 |
Common stock equities | Pension Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 228,088 | 235,109 |
Common stock equities | Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 228,088 | 235,109 |
Common stock equities | Other Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 196,153 | 243,741 |
Common stock equities | Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 196,153 | 243,741 |
Mutual funds | Pension Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 233,732 | 251,506 |
Mutual funds | Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 233,732 | 251,506 |
Mutual funds | Other Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Fair value of plan assets | 39,269 | 67,418 |
Mutual funds | Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 39,269 | 67,418 |
Equities | Pension Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 408,763 | 266,840 |
Fair value of plan assets | 408,763 | 266,840 |
Equities | Other Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 75,310 | 95,814 |
Fair value of plan assets | 75,310 | 95,814 |
Real estate | Pension Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 171,569 | 161,449 |
Fair value of plan assets | 171,569 | 161,449 |
Real estate | Other Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 15,422 | 14,509 |
Fair value of plan assets | 15,422 | 14,509 |
Fixed income securities | Pension Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 90,869 |  |
Fair value of plan assets | 90,869 |  |
Partnerships | Pension Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 133,379 | 208,915 |
Fair value of plan assets | 133,379 | 208,915 |
Partnerships | Other Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other |  | 3,060 |
Fair value of plan assets |  | 3,060 |
Short-term investments and other | Pension Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 98,505 | 112,337 |
Fair value of plan assets | 99,713 | 112,337 |
Short-term investments and other | Pension Benefits | Significant Other Observable Inputs (Level 2) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 1,208 |  |
Short-term investments and other | Other Benefits | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Other | 9,178 | 9,851 |
Fair value of plan assets | 20,595 | $ 9,851 |
Short-term investments and other | Other Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | 11,268 |  |
Short-term investments and other | Other Benefits | Significant Other Observable Inputs (Level 2) | Pinnacle West |  |  |
Fair value of Pinnacle West's pension plan and other postretirement benefit plan assets, by asset category |  |  |
Gross fair value of plan assets | $ 149 |  |

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