Palo Verde Operations and Maintenance Expense (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Jointly Owned Utility Plant Interests [Line Items]
Utilities Operating Expense, Maintenance and Operations	$ 80,160	$ 79,187	$ 321,254	$ 317,518
Palo Verde Generating Station
Jointly Owned Utility Plant Interests [Line Items]
Utilities Operating Expense, Maintenance and Operations	$ 22,175	$ 21,608	$ 99,931	$ 96,179

Amount of operating expense for routine plant maintenance, repairs and operations of regulated operation.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2(b))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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Period Type:	duration