UTILITY PLANT AND JOINTLY-OWNED FACILITIES (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 MWh	Mar. 31, 2020 MWh rice	Dec. 31, 2019 USD ($) rice	Dec. 31, 2018 USD ($)
Public Utility, Property, Plant and Equipment [Line Items]
Public utilities, plant in service			$ 4,534,896	$ 4,160,640
Plant in Service			7,118,867	$ 6,528,069
Gila River Unit 2
Public Utility, Property, Plant and Equipment [Line Items]
Term of contract	20 years
Generating capacity (MW) | MWh	550
Option to purchase unit, term	3 years
Public utilities, plant in service			$ 165,000
RICE Units
Public Utility, Property, Plant and Equipment [Line Items]
Number of units in service | rice			5
Number of units | rice			10
Plant in Service			$ 82,000
RICE Units | Forecast
Public Utility, Property, Plant and Equipment [Line Items]
Generating capacity (MW) | MWh		188
Number of units | rice		5