Palo Verde Operations and Maintenance Expense (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Palo Verde Generating Station
Operations & Maintenance Expenses [Line Items]
Utilities Operating Expense, Maintenance and Operations	$ 21,608	$ 22,343	$ 96,179	$ 98,134

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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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration