Utility Plant, Palo Verde and Other Jointly-Owned Utility Plant (Summary of Jointly Owned Utility Plants) (Details) $ in Thousands	12 Months Ended
Dec. 31, 2015 USD ($) utilityplantunit	Dec. 31, 2014 USD ($)
Jointly Owned Utility Plant Interests [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	7.00%
Palo Verde Generating Station
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
Palo Verde Participants
Jointly Owned Utility Plant Interests [Line Items]
Jointly Owned Plant, Number of Participants	6
Four Corners Generating Station
Jointly Owned Utility Plant Interests [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	7.00%
Other Jointly Owned Utlity Plant
Jointly Owned Utility Plant Interests [Line Items]
Electric plant in service	$ 229,627	$ 219,318
Accumulated depreciation	(181,886)	(176,492)
Construction work in progress	9,528	6,900
Total	57,269	49,726
Palo Verde \| Jointly Owned Nuclear Power Plant
Jointly Owned Utility Plant Interests [Line Items]
Electric plant in service	917,483	874,817
Accumulated depreciation	(304,060)	(286,585)
Construction work in progress	48,938	55,632
Total	$ 662,361	$ 643,864

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://www.xbrl.org/2003/role/presentationRef
-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=6500719&loc=d3e659983-123033

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 10
-Section C

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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.

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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

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- Definition

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

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- Definition

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

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- Definition

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

+ References

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