FAIR VALUE MEASUREMENTS (Level 3 Measurements and Sensitivity Analysis) (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) $ / MWh	Dec. 31, 2017 USD ($) $ / MWh
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value	$ 4,137	$ 4,145
Liabilities, Fair Value	88	88
Congestion revenue rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value	110	129
Liabilities, Fair Value	$ 44	$ 24
Congestion revenue rights | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range | $ / MWh	(18.61)	(16.03)
Congestion revenue rights | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range | $ / MWh	32.26	11.99
Power purchase agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value	$ 0	$ 0
Liabilities, Fair Value	$ 42	$ 63
Power purchase agreements | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range | $ / MWh	19.81	18.81
Power purchase agreements | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range | $ / MWh	38.80	38.80