Utility Plant, Palo Verde and Other Jointly-Owned Utility Plant (Summary of Jointly Owned Utility Plants) (Details) $ in Thousands	12 Months Ended
Dec. 31, 2018 USD ($) utilityplantunit	Dec. 31, 2017 USD ($)
Palo Verde Participants
Jointly Owned Utility Plant Interests [Line Items]
Jointly Owned Plant, Number of Participants	6
Jointly Owned Nuclear Power Plant
Jointly Owned Utility Plant Interests [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	15.80%
Jointly Owned Utility Plant, Number of Units \| utilityplantunit	3
Electric plant in service	$ 1,024,771	$ 994,075
Accumulated depreciation	(358,606)	(338,699)
Construction work in progress	44,719	40,946
Total	710,884	696,322
Transmission
Jointly Owned Utility Plant Interests [Line Items]
Electric plant in service	94,155	97,603
Accumulated depreciation	(75,096)	(72,822)
Construction work in progress	1,511	1,014
Total	$ 20,570	$ 25,795

Jointly Owned Plant, Number of Participants

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- Definition

Jointly Owned Utility Plant, Number of Units

+ References

No definition available.

+ Details

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- Definition

The period end amount owned by the reporting utility of a jointly owned utility plant in service.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-URI http://asc.fasb.org/extlink&oid=115995737&loc=d3e659983-123033

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

The total period end amount of plant in service and construction in progress net of accumulated depreciation for the utility's share of jointly owned electricity generation plant.

+ References

+ Details

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- Definition

The period end amount of construction work in progress at a jointly owned electricity generation plant.

+ References

+ Details

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- Definition

The period end amount of accumulated depreciation in a jointly owned electricity generation plant.

+ References

+ Details

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- Definition

The proportionate share of interest (percent) in a jointly owned utility plant.

+ References

+ Details

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