FAIR VALUE MEASUREMENTS (Level 3 Measurements and Sensitivity Analysis) (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 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	$ 4,159	$ 4,145
Liabilities, Fair Value	80	88
Congestion revenue rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value	113	129
Liabilities, Fair Value	32	24
Power purchase agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value	0	0
Liabilities, Fair Value	$ 47	$ 63
Minimum | Congestion revenue rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range | $ / MWh	(18.61)	(16.03)
Minimum | Power purchase agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range | $ / MWh	18.81	18.81
Maximum | Congestion revenue rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range | $ / MWh	32.26	11.99
Maximum | Power purchase agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range | $ / MWh	38.80	38.80