FAIR VALUE MEASUREMENTS AND DERIVATIVE INSTRUMENTS (Level 3 Fair Value Measurements) (Details) - Level 3 - Market approach - Forward Power Contracts $ in Millions	Dec. 31, 2017 USD ($) $ / megawatt_hour	Dec. 31, 2016 USD ($) $ / megawatt_hour
Fair Value Inputs Assets Liabilities Quantitative Information [Line Items]
Derivative Assets | $	$ 3	$ 2
Derivative Liabilities | $	$ (1)	$ (1)
Minimum
Fair Value Inputs Assets Liabilities Quantitative Information [Line Items]
Market Price | $ / megawatt_hour	17.65	20.90
Maximum
Fair Value Inputs Assets Liabilities Quantitative Information [Line Items]
Market Price | $ / megawatt_hour	34.60	40.00