Commitments and Contingencies - Superfund-Related Matters, Southwest Power Outage and Clean Air Act (Details) - APS - Contaminated groundwater wells $ in Millions			6 Months Ended
Dec. 16, 2016 plaintiff	Aug. 06, 2013 Defendant	Jun. 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 \| plaintiff	2

Costs related to site-wide groundwater remedial investigation and feasibility study work plan because subsidiary has been found to be potentially responsible parties ("PRPs") with liability for cleanup of hazardous substances that contaminates the soil, water or air, under the Comprehensive Environmental Response, Compensation and Liability Act ("Superfund").

No definition available.

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