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Regulatory Matters - Four Corners, Cholla and Navajo Plant (Details) - APS - USD ($) $ in Millions |  |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Dec. 23, 2014 | Dec. 30, 2013 | Jun. 30, 2016 | Dec. 31, 2017 | Dec. 31, 2015
Retired power plant costs |  |  |  |  |
Acquisition |  |  |  |  |
Regulatory asset, net book value |  |  |  | $ 105.0 |
Navajo Plant |  |  |  |  |
Acquisition |  |  |  |  |
Regulatory asset, net book value |  |  |  | 99.0 |
SCE | Four Corners |  |  |  |  |
Acquisition |  |  |  |  |
Regulatory assets |  |  |  |  | $ 12.0
Regulatory assets, write of amount |  |  | $ 12.0 |  |
Four Corners Units 4 and 5 | SCE |  |  |  |  |
Acquisition |  |  |  |  |
Ownership interest acquired (as a percent) |  | 48.00% |  |  |
Settlement agreement, ACC approved rate adjustment, annualized customer impact | $ 57.1 |  |  |  |
Transmission termination agreement net receipt due to negotiation of alternate arrangement |  | $ 40.0 |  |  |
Four Corners Units 4 and 5 | SCE | Four Corners cost deferral |  |  |  |  |
Acquisition |  |  |  |  |
Regulatory assets |  |  |  | $ 56.0 |
Amortization period |  |  |  | 10 years |

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