|**Long-Term Debt and Liquidity Matters (Tables)**                                | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                   
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|**Debt Disclosure [Abstract]**                                                   |                                                                                                                                                                                                                                                                                                                                                                  
|Schedule of estimated fair value of long-term debt, including current maturities | The following table presents the estimated fair value of our long-term debt, including current maturities (dollars in thousands):As of September 30, 2018As of December 31, 2017CarryingAmountFair ValueCarryingAmountFair ValuePinnacle West$298,640$292,185$298,421$298,608APS4,788,7244,900,2104,573,2925,006,348Total$5,087,364$5,192,395$4,871,713$5,304,956
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|                                                                                 |                                                                                                                                                                                                                                                                                                                                                                  
|                                                                                 | As of September 30, 2018                                                                                                                                                                                                                                                                                                                                         
|                                                                                 | CarryingAmount                                                                                                                                                                                                                                                                                                                                                   
|Pinnacle West                                                                    | $                                                                                                                                                                                                                                                                                                                                                                
|APS                                                                              | 4,788,724                                                                                                                                                                                                                                                                                                                                                        
|Total                                                                            | $                                                                                                                                                                                                                                                                                                                                                                

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