Commitments and Contingencies - Superfund-Related Matters, Southwest Power Outage and Clean Air Act (Details) - APS - Contaminated groundwater wells $ in Millions				9 Months Ended
	Apr. 05, 2018 plaintiff	Dec. 16, 2016 plaintiff	Aug. 06, 2013 Defendant	Sep. 30, 2018 USD ($)
Loss Contingencies [Line Items]
Costs related to investigation and study under Superfund site | $				$ 2
Number of defendants against whom Roosevelt Irrigation District (RID) filed lawsuit | Defendant			24
Number of plaintiffs		2
Settled Litigation
Loss Contingencies [Line Items]
Number of plaintiffs	2