FAIR VALUE MEASUREMENTS (Level 3 Measurements and Sensitivity Analysis) (Details) $ in Millions	Dec. 31, 2018 USD ($) $ / MWh	Dec. 31, 2017 USD ($) $ / MWh
Market approach \| Congestion revenue rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets \| $	$ 203	$ 129
Liabilities \| $	75	24
Discounted cash flow \| Power purchase agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets \| $	0	0
Liabilities \| $	$ 33	$ 63
CRR auction prices \| Market approach \| Congestion revenue rights \| Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range (in dollars per mwh) \| $ / MWh	(18.61)	(16.03)
CRR auction prices \| Market approach \| Congestion revenue rights \| Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range (in dollars per mwh) \| $ / MWh	32.26	11.99
Forward prices \| Discounted cash flow \| Power purchase agreements \| Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range (in dollars per mwh) \| $ / MWh	19.81	18.81
Forward prices \| Discounted cash flow \| Power purchase agreements \| Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range (in dollars per mwh) \| $ / MWh	38.80	38.80

Value of input used to measure net derivative asset (liability).

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