REGULATORY MATTERS (Federal Tax Legislation) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Regulatory Assets [Roll Forward]
Regulatory Deferral	$ 4,259	$ 1,707
Arizona Corporation Commission
Regulatory Assets [Roll Forward]
Customers bill as percent of returned savings	50.00%
Deferred TCJA Customer Refunds, Liability	$ 12,000			$ 8,000
Arizona Corporation Commission | Scenario, Forecast
Regulatory Assets [Roll Forward]
Change in tax rate, refund to customers, net of amortization			$ 35,000
Arizona Corporation Commission | Revenue Refund
Regulatory Assets [Roll Forward]
Beginning of Period	0	4,000	$ 0
ACC Approved Refund (Reduction in Operating Revenues)	(7,000)	(7,000)
Amount Returned to Customers Through Bill Credits	3,000	4,000
Regulatory Deferral	4,000	2,000
End of Period	$ 0	$ 3,000