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FAIR VALUE MEASUREMENTS AND DERIVATIVE INSTRUMENTS (Schedule of Reconciliation of Changes in Fair Value of Assets and Liabilities) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Roll-forward    
Beginning of Period $ 2 $ 1
Gains (Losses) Recorded    
Regulatory Assets or Liabilities, Derivative Instruments (4) 1
Operating Revenues 5 4
Settlements (2) (4)
End of Period 1 2
Gains (Losses), Assets (Liabilities) Still Held $ 1 $ 2