Utility Plant, Palo Verde and Other Jointly-Owned Utility Plant Palo Verde Operations and Maintenance Expense (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Jointly Owned Utility Plant Interests [Line Items]
Operations and maintenance	$ 332,557	$ 334,883	$ 320,281
Palo Verde Generation Station
Jointly Owned Utility Plant Interests [Line Items]
Operations and maintenance	$ 95,525	$ 96,454	$ 99,364

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