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