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Regulatory Matters - Four Corners and Cholla (Details) - APS - USD ($) $ in Millions |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Dec. 23, 2014 | Dec. 30, 2013 | Sep. 30, 2018 | Apr. 30, 2018 | Jun. 30, 2016 | Sep. 30, 2018 | Dec. 31, 2015
SCE | Four Corners Units 4 and 5 |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Ownership interest acquired |  | 48.00% |  |  |  |  |
Settlement agreement, ACC approved rate adjustment, annualized customer impact | $ 57.1 |  | $ 58.5 | $ 67.5 |  |  |
Net receipt due to negotiation of alternate arrangement |  | $ 40.0 |  |  |  |  |
Four Corners cost deferral | SCE | Four Corners Units 4 and 5 |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Regulatory assets, non-current |  |  | 50.0 |  |  | $ 50.0 |
Regulatory noncurrent asset amortization period |  |  |  |  |  | 10 years |
Retired power plant costs |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Net book value |  |  | 93.0 |  |  | $ 93.0 |
Navajo Plant |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Net book value |  |  | $ 90.0 |  |  | $ 90.0 |
Four Corners | SCE |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Regulatory assets, non-current |  |  |  |  |  |  | $ 12.0
Regulatory asset, write off amount |  |  |  |  | $ 12.0 |  |

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