Palo Verde Generating Station Operations and Maintenance Expense (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Jointly Owned Utility Plant Interests [Line Items]
Utilities Operating Expense, Maintenance and Operations	$ 80,704	$ 80,413	$ 332,848	$ 335,136
Palo Verde Generating Station
Jointly Owned Utility Plant Interests [Line Items]
Utilities Operating Expense, Maintenance and Operations	$ 19,613	$ 21,344	$ 93,794	$ 95,623

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2(b))
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration