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Summary of Significant Accounting Policies - Narrative (Details) $ / shares in Units, $ in Millions | 1 Months Ended | 12 Months Ended | 36 Months Ended |
--- | --- | --- | --- | ---
May 31, 2014 $ / kWh | Dec. 31, 2018 USD ($) $ / shares shares | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2017 | Jul. 06, 2016
Approximate remaining average useful lives of utility property |  |  |  |  |  |
Depreciation |  | $ 486 | $ 453 | $ 422 |  |
Depreciation rates (as a percent) |  | 2.81% | 2.80% | 2.66% |  |
Allowance for Funds Used During Construction |  |  |  |  |  |
Composite rate used to calculate AFUDC (as a percent) |  | 7.03% | 6.68% | 7.17% |  |
Income Taxes |  |  |  |  |  |
Percent likelihood largest tax benefit amount is realized (greater than) |  | 50.00% |  |  |  |
Intangible Assets |  |  |  |  |  |
Amortization expense |  | $ 68 | $ 72 | $ 58 |  |
Estimated amortization expense on existing intangible assets over the next five years |  |  |  |  |  |
2019 |  | 58 |  |  |  |
2020 |  | 47 |  |  |  |
2021 |  | 34 |  |  |  |
2022 |  | 25 |  |  |  |
2023 |  | $ 22 |  |  |  |
Remaining amortization period for intangible assets |  | 8 years |  |  |  |
Pinnacle West |  |  |  |  |  |
Preferred Stock |  |  |  |  |  |
Preferred stock, shares authorized (in shares) | shares |  | 10,000,000 |  |  |  |
ARIZONA PUBLIC SERVICE COMPANY |  |  |  |  |  |
Nuclear Fuel |  |  |  |  |  |
Charges for the permanent disposal of spent nuclear fuel (in dollars per kWh) | $ / kWh | 0.001 |  |  |  |  |
Preferred Stock |  |  |  |  |  |
Preferred stock, shares authorized (in shares) | shares |  | 15,535,000 |  |  |  |
Preferred stock par or stated value per share 1 (in dollars per share) | $ / shares |  | $ 25 |  |  |  |
Preferred stock par or stated value per share 2 (in dollars per share) | $ / shares |  | 50 |  |  |  |
Preferred stock par or stated value per share 3 (in dollars per share) | $ / shares |  | $ 100 |  |  |  |
Minimum |  |  |  |  |  |
Approximate remaining average useful lives of utility property |  |  |  |  |  |
Depreciation rates (as a percent) |  |  |  |  | 0.18% |
Maximum |  |  |  |  |  |
Approximate remaining average useful lives of utility property |  |  |  |  |  |
Depreciation rates (as a percent) |  |  |  |  | 19.67% |
Investments |  |  |  |  |  |
Ownership percentage for classification as cost method investments by El Dorado |  | 20.00% |  |  |  |
Fossil plant |  |  |  |  |  |
Approximate remaining average useful lives of utility property |  |  |  |  |  |
Average useful life |  | 17 years |  |  |  |
Nuclear plant |  |  |  |  |  |
Approximate remaining average useful lives of utility property |  |  |  |  |  |
Average useful life |  | 23 years |  |  |  |
Other generation |  |  |  |  |  |
Approximate remaining average useful lives of utility property |  |  |  |  |  |
Average useful life |  | 19 years |  |  |  |
Transmission |  |  |  |  |  |
Approximate remaining average useful lives of utility property |  |  |  |  |  |
Average useful life |  | 39 years |  |  |  |
Distribution |  |  |  |  |  |
Approximate remaining average useful lives of utility property |  |  |  |  |  |
Average useful life |  | 34 years |  |  |  |
General plant |  |  |  |  |  |
Approximate remaining average useful lives of utility property |  |  |  |  |  |
Average useful life |  | 6 years |  |  |  |
El Paso's Interest in Four Corners | 4CA |  |  |  |  |  |
Utility Plant and Depreciation [Line Items] |  |  |  |  |  |
Ownership interest acquired (as a percent) |  | 7.00% |  |  |  | 7.00%

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