FAIR VALUE MEASUREMENTS AND DERIVATIVE INSTRUMENTS (Schedule of Reconciliation of Changes in Fair Value of Assets and Liabilities) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract]
Beginning of Period	$ 1,000,000	$ (2,000,000)
Gains (Losses) Recorded to:
Regulatory Assets or Liabilities, Derivative Instruments	1,000,000	2,000,000
Wholesale Revenues	4,000,000	4,000,000
Settlements	(4,000,000)	(3,000,000)
End of Period	2,000,000	1,000,000
Gains (Losses), Assets (Liabilities) still held	2,000,000	1,000,000
Transfers out of Level 3	$ 0	$ 0