Palo Verde Operations and Maintenance Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Jointly Owned Utility Plant Interests [Line Items]
Utilities Operating Expense, Maintenance and Operations	$ 82,186	$ 83,355	$ 243,907	$ 252,370	$ 326,420	$ 336,506
Palo Verde Generation Station
Jointly Owned Utility Plant Interests [Line Items]
Utilities Operating Expense, Maintenance and Operations	$ 20,713	$ 20,277	$ 66,294	$ 67,429	$ 95,319	$ 98,813