REGULATORY MATTERS (Federal Tax Legislation) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Regulatory Asset (Liability) [Roll Forward]
Beginning of Period	$ 400	$ 388
End of Period		$ 400
Regulatory commission, decrease in transmission rate		5.30%
Arizona Corporation Commission
Public Utilities, General Disclosures [Line Items]
Refund to customers, after the EDIT amortization true-up		$ 33
Federal Energy Regulatory Commission
Regulatory Asset (Liability) [Roll Forward]
ACC Approved Refund (Reduction in Operating Revenues)		1
Regulated operating revenue		(1)
Scenario, Forecast | Arizona Corporation Commission
Regulatory Asset (Liability) [Roll Forward]
Amount Returned to Customers Through Bill Credits	34
Revenue Refund | Arizona Corporation Commission
Regulatory Asset (Liability) [Roll Forward]
Beginning of Period	$ 4	0
ACC Approved Refund (Reduction in Operating Revenues)		33
Amount Returned to Customers Through Bill Credits		37
End of Period		4
Regulated operating revenue		$ (33)