NATURE OF OPERATIONS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Allowance for Doubtful Accounts) (Details) - Allowance for Doubtful Accounts - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning of Period	$ 5	$ 27	$ 5
Additions Charged to Cost and Expense	3	4	2
Write-offs	(3)	(3)	(3)
End of Period	5	5	27
Springerville Unit 1 Third Party Owner Allegation
Movement in Valuation Allowances and Reserves [Roll Forward]
Provision for Springerville Unit 1, Third-Party Owners	$ 0	$ (23)	$ 23