Palo Verde Operations and Maintenance Expense (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Palo Verde Generating Station
Operations & Maintenance Expenses [Line Items]
Utilities Operating Expense, Maintenance and Operations	$ 20,441	$ 21,123	$ 67,980	$ 67,514	$ 97,380	$ 97,451

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

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