REGULATORY MATTERS (Federal Tax Legislation) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets [Roll Forward]
Regulatory Deferral	$ 7,705	$ (1,562)	$ 0
Arizona Corporation Commission
Public Utilities, General Disclosures [Line Items]
Change in tax rate, refund to customers, net of amortization	$ 35,000
Change in tax rate, customers bill as percent of returned savings	50.00%
Arizona Corporation Commission | Revenue Refund
Regulatory Assets [Roll Forward]
Beginning of Period	$ 4,000	0
ACC Refund (Reduction in Operating Revenues)	(34,000)	(33,000)
Amount Returned to Customers through Bill Credits	22,000	37,000
Regulatory Deferral	8,000	0
End of Period	$ 0	4,000	$ 0
FERC
Regulatory Assets [Roll Forward]
ACC Refund (Reduction in Operating Revenues)		$ (1,000)
Regulatory commission, decrease in transmission rate		5.30%