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FAIR VALUE MEASUREMENTS AND DERIVATIVE INSTRUMENTS (Reconciliation of Changes in Fair Value of Assets and Liabilities) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]        
Beginning of Period $ 5 $ 4 $ 2 $ 1
Gains (Losses) Recorded        
Regulatory Assets or Liabilities, Derivative Instruments 1 1 1 3
Operating Revenues 0 0 4 4
Settlements (6) (1) (7) (4)
End of Period 0 4 0 4
Gains (Losses), Assets (Liabilities) Still Held $ (2) $ 0 $ 0 $ 4