NATURE OF OPERATIONS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Allowance for Doubtful Accounts) (Details) - Allowance for Doubtful Accounts [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning of Period	$ 27	$ 5	$ 5
Additions Charged to Cost and Expense	4	2	2
Write-offs	(3)	(3)	(2)
End of Period	5	27	5
Springerville Unit 1 Third Party Owner Allegation [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Adjustments	$ (23)	$ 23	$ 0