FAIR VALUE MEASUREMENTS (Level 3 Measurements and Sensitivity Analysis) (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) $ / MWh	Dec. 31, 2018 USD ($) $ / MWh
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	$ 7,477	$ 5,350
Liabilities	$ 85	$ 109
Congestion revenue rights | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable input (dollars per mwh) | $ / MWh	(13.11)	(18.61)
Congestion revenue rights | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable input (dollars per mwh) | $ / MWh	22.76	32.26
Power purchase agreements | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable input (dollars per mwh) | $ / MWh	19.68	19.81
Power purchase agreements | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable input (dollars per mwh) | $ / MWh	38.80	38.80
Market approach | Congestion revenue rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	$ 191	$ 203
Liabilities	64	75
Discounted cash flow | Power purchase agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	1	0
Liabilities	$ 19	$ 33